Category Archives: Weapons Technology

How Did the FG-42 Selector Work?

We were asked that yesterday and we pontifically pronounced, “it fired from the open bolt in automatic mode, and from the close bolt in semi.”

This one's an SMG Guns semi clone. Pretty, though, innit?

This one’s an SMG Guns semi clone. Pretty, though, innit? Images do embiggen with a click.

Then we rested back on our laurels as Gun Expert and —

“Well, how did they make it do that?”

“*!” Hmm… How did they? “Let me get back to you on that.”

Fortunately, several references on the shelves explain it in terms our walnut sized brain could grasp. It turns out it was very simple, when you consider how complex some of the other design options made the FG. And it imposed some trade-offs, costing the rifle significant semi-auto accuracy as the price of that mechanical simplicity. Let’s walk you through it.

It worked exactly the same on the First and Second model of the FG, by the way; so we will use images of both in this post.

FG42-0034- grip FW

This image is from a crudely DEWATted Second Model FG that was examined by Forgotten Weapons. There’s a great set of images there, and the gun’s internals are mostly present and correct.

The selector switch is on the left side of what we’d call the grip frame. (The German manuals call this part the Lager which can mean holder or receiver, too, but we’ll stick with “grip frame”). The selector swings through 180º of travel; knob forward covers an “E” for Einzelfeuer (“single fire,” semi-auto), and knob rear clicks on to “D” for Dauerfeuer, (“continuous fire,” automatic). Note that the letter that shows is the antonym of the function you get. Don’t ask us; Hermann Göring was not available to take complaints.

FG-42 exploded view

Comparing the Bedienungsanleitung (manual) image of a First Model to the photo of the second model above that, we can see how the trigger works. The trigger pivots on a pin forward of, and slightly below, the selector switch. The axis of the selector switch is also the axle of the sear (in the diagram, Part B8 Abzughebel, literally “trigger lever”). The sear nose (Fangnase, “catch nose,” B8a) is the hardened end of the sear that engages a notch (if you learned engineering English in Britain, a “bent”) in the operating rod (Verschlußführungsstück, “bolt guiding piece,” Part D10).

There are, however, two notches in the op-rod. One is towards the front end, and mostly right of center. One is towards the tail end, and mostly left of center. You can make out the two notches in this Forgotten Weapons photo.

FG42-0003_FWRotating the selector moves the sear laterally either right to align with the front-end notch, or left to align with the tail-end notch. If it aligns with the tail-end notch, a disconnector (Unterbrecher, literally “interrupter”, B9), works by disengaging the trigger from the sear until the trigger is released (i.e., normal semi-auto trigger reset). Thus the selector engages the sear nose with either the nose-end notch, which holds the op rod and bolt assembly to the rear, or the tail-end notch, which holds the op rod and firing pin only to the rear, allowing the bolt to lock fully into battery.

Releasing the trigger releases the op-rod, then. If the weapon is on full automatic, the bolt and op-rod come forward, the bolt locks, the op-rod finishes its full travel, and the firing pin initiates the cartridge. The whole thing cycles again and continues to do so until the operator releases the trigger. When he does, the bolt is held in automatic battery — to the rear.

These schematics are from Allson & Toomey's Small Arms, pp. 226-227.

These schematics are from Allsop & Toomey’s Small Arms, pp. 226-227. The depiction of the selector in these drawings is how we came to understand that the selector (“change lever” in British English) covers the appropriate letter for type of fire selected.

If the weapon is on semi (selector knob swung 180º to the front), the trigger releases the op-rod, which brings the firing pin down on the primer. The bolt then cycles, but returns to semi-auto battery, closed bolt on a live cartridge, regardless of trigger position. The disconnector rides in the notch forward of the rear notch (here “bent”) only to disconnect when in Semi.


If you’re feeling envious of FG-42s, you can buy an excellent semi repro from SMG Guns, you can pay more than a new luxury car for a transferable, or you can take the following image, a pile of steel, wood and aluminum, and a set of files and try to do what SMG did:

FG-42 Type II exploded view

It may take a while. Best of luck to you!

Now, the FG42 wasn’t the last word in open/closed bolt hybrid firing mechanisms. As mentioned, having the whole op rod and firing pin move was inimical to accuracy. This not only increased the motion of the firearm on firing, but it increased lock time substantially, giving that motion more time to work on sending your projectiles wild. But that was a tradeoff that designers at Rheinmettal accepted for their simple and reliable open/closed bolt mechanism.

As we’ve seen, waste heat is a real killer of combat weapons in automatic fire, and by extension, a potential killer of the men who fire them. Firing from an open bolt reduces the incremental temperature increase per automatic round fired, by allowing more air to circulate and more of the potential radiative area to be exposed to ambient-temperature cooling air. This has the side effect of moving the critical temperature area or point further up the barrel from its usual position 5 to 8 inches in front of the chamber.

Firing from an open bolt also prevents cook-offs. Contrary to common misconception, cook-offs are usually not instantaneous but result from a round remaining chambered in a hot barrel for some seconds or minutes. For a cook-off to be instantaneous (and risk an out-of-battery ignition) the temperature has to be extremely elevated. For a routine cook-off, which can take some time to happen, the biggest danger is that no one is expecting the weapon to fire, and people may be in an unsafe position forward of its muzzle at that point.

The FG42 was a remarkably good weapon, like many WWII German weapons. Not good enough for them to win the war, fortunately; it was the very devil to produce (ask Steve at SMG!) and was produced in the sort of numbers that would be a rounding error, or the scrappage involved in training some new line workers, in American, British or Russian production. The US produced, for example, about 40 times as many BARs as Germany produced FG42s; Russian production of the pan-fed DP28 LMG was easily double that. (German production wasn’t as dismal as you might think. They produced more rifles and carbines of all types than the USA did. But they did have a tendency to engineer something very good, and then fail to build it in numbers that would make a difference).

A Taxonomy of Safeties

There are several kinds of safeties that are used on service weapons to ensure that only the proper and deserving people are shot. They generally interface in some way with the firing mechanism of the firearm. They may act on the trigger, the hammer or striker, or the sear, or (in some fiendishly clever arrangements) more than one of the above. It is generally thought better to positively lock the striker or firing pin than merely to lock the sear or trigger. If the mechanism fails due to parts breakage, it is easier to design a fail-safe mechanism if the striker or firing pin is immobilized.

Safeties Classified by Operator Volition

Safeties can be classified based on the degree of volition required to use them. An applied safety must be consciously put on, in most cases. An automatic safety is unconsciously applied as the pistol is taken up. Examples of automatic safeties include:

  1. the Glock Safe Action trigger and its many copies and derivatives;
  2. the grip safeties characteristic of many Browning designs, such as the M1911 .45 and the FN M1910 pocket pistol;
  3. similar grip safeties on open-bolt submachine guns such as the Madsen and the Uzi. (An open-bolt SMG poses peculiar safety problems);
  4. transfer-bars and other means to ensure a weapon can’t fire unless the trigger is pulled;
  5. mechanisms that hold a firing pin back until a weapon with a locking breech is fully in battery (the disconnector often does double-duty as this part);
  6. Firing-pin immobilizers as in the Colt Series 80 and newer M1911s (an earlier firing pin safety, the Swartz Safety, was used in commercial Colt 1911s from circa 1937 to 1940, and is used by Kimber today);
  7. A heavy, smooth trigger pull such as that on a traditional Double Action revolver or a DA/SA autopistol can prevent unintentional discharges. However, some heavy triggers (like the Glock NY2) have a bad enough effect on accuracy as to threaten bystanders with unintentional shooting.
  8. Magazine safeties, an obsolete European concept;
  9. Half-cock notches (in British/European English usage, these may be called half-cock “bents.”)

Contrasting with these automatic safeties, that do their work without conscious application by the operator, there are Applied or volitional safeties. Applied Safeties are usually classified by what part of the firing mechanism they work on, and so examples of Applied safeties break down into:

  1. Safeties that lock the trigger. The simplest of these are the crude trigger-blocking safeties on an SKS or Tokarev SVT. More complex trigger-locking safeties are found in the AR series of rifles and the FN-FAL;
  2. Safeties that lock the firing mechanism (which may be further divided into those that lock the firing pin, like the Walther P.38 or Beretta M92, and those that lock the hammer, like the US M1 Rifle, or
  3. The bolt holding notch in many 2nd-generation submachine guns. (These are reminiscent in a way of the safety of the Mosin-Nagant rifle, which requires the cocking piece to be rotated and caught in a notch). The case can be made that this is a firing mechanism lock, because the bolt with its fixed firing pin is the firing mechanism.
  4. Safeties that lock the sear. Examples include the .45 M1911, its younger brother the BHP, many other auto pistols, and most general purpose machine guns. Some require the weapon to be cocked to lock the sear, others allow locking the bolt forward (the RPD LMG and the Sterling SMG are examples of this).
  5. Safeties that disconnect the trigger from the sear. This is found in the Bren gun and many other Czech designs, historically. The ZB 26 and its derivatives were quite cunning: in one position, the selector brings the trip lever to engage the semi notch, which is in the upper side of a window in the sear. In the other position, it engages the auto notch in the lower side. In the intermediate, “safe,” position, the  trip lever clears both notches and the weapon does not fire.

Note that automatic safeties, too, can be broken down as working on the trigger, the firing mechanism, and the sear, also. So safeties can also be Classified by Operation.

Safeties Classified by Operation

It is possible to classify safeties in the first place by their means of action:

  1. Trigger safeties
  2. Firing-mechanism (striker, hammer, firing pin) safeties
  3. Sear safeties
  4. Disconnecting safeties.

This is true, obviously, for both automatic and volitional safeties, and classifying them this way puts their mode of action forward as more important than their mode of engagement, which (applied/volitional or automatic) becomes a secondary trait.

One More Trait: Must the Firearm be Cocked?

It is only possible to engage many safeties when the weapon is cocked or ready to fire (presuming a chambered round). Familiar examples include the AR series rifles and the 1911 pistol and other Browning hammer designs. Other safeties engage regardless of the energy state of the striker or hammer, for example the AK, the Remington Model 8 (a Browning-designed trigger mechanism that was deeply influential on 20th and 21st Century firearms designers, including Garand, Kalashnikov and Stoner), and the RPD light machine gun.

Combination Safeties

While a weapon may have multiple safeties that do different things (or multiple modes that engage the same safety, as in the safety lever and grip safety of early Lugers), it’s possible for a single cunningly-designed safety to disable multiple points of the firing chain at once. For instance, the Lee-Enfield safety is a model of versatility: it locks the striker, locks the bolt closed (preventing the chambering of a round), and disconnects the striker from the sear. The M1911 or Browning High-Power safety locks the slide closed as well as locks

It’s also possible for a volitional safety to be combined with other functions. The most common example of this is the combined safety/selector switch of most modern assault rifles, like the M16 or AK-47.

To Sum Up

There are a great but finite number of ways to design safety features on modern firearms. Careful study of prior art allows today’s designer truly to stand on the shoulders of the giants in the field. John Browning left no memoir or technical book, nor did John Garand, John D. Pedersen, Gene Stoner; and the many memoirs of Mikhail Kalashnikov are disappointing to the technical reader. But each of these geniuses spoke to us in the art of his designs, and they are still available for us to study and to try to read what their art is trying to tell us.

We have not, in this limited post, attempted to discuss “best practices” or the pros and cons of any individual safety design. Very often, the designer will be limited by the customer’s instructions or specifications. (For example, the grip safety of the 1911, which 1970s and 80s custom smiths often pinned in engagement as a potential point of combat failure, was requested of John M. Browning by the US Cavalry. The other military branches didn’t feel such a need, but the horse soldiers did, and Browning first added it on his .38 caliber 1902 Military pursuant to a similar request). Thus, even as a designer, your safety design decisions may not be your own.

Notes and Sources

  • This post has been modified since it was first posted, to expand it.
  • This post will be added to The Best of WeaponsMan Gun Tech.

This post owes a great deal to the following work:

Allsop, DF, and Toomey, MA. Small Arms: General Design. London: Brassey’s, 1999.

Chapter 13 is an extensive review of trigger mechanisms, including safeties, and while their classification of safeties is different from ours, their explanations are clear and concise.

Thanks to the commenters who not only recommend this long out-of-print book, but also sent us a link to a bookstore that had it (it’s a copy withdrawn from a military library, as it turns out). This out-of-print work is less technical and deep, but considerably more modern, than Balleisen; its examples are primarily British.


GhostGunner Update

It looks as if Defense Distributed’s original intent, to ship the initial batch of GhostGunners by Christmas, ran into the buzzsaw called reality, and the machines did not ship on time.

The [GhostGunner] team worked all through December to begin fulfillment just before Christmas Day, but due to two of our US suppliers missing their original and revised December delivery deadlines, we have been at the mercy of factors outside of our control.

The bottleneck came down to our steel enclosures.


Though we are now delayed over ten days in our internal shipping schedule, we are fully prepared to begin fulfillment when we receive sufficient enclosures, which should be as early as the first week of January.

It’s not ideal news, but we are fully staffed for final assemblies and have run a really great catchup game in our V&V.

The delays have allowed some improvements to the machine. They’ve brought circuit-board production in-house:

Final GG software testing continues uninhibited and we’ve established our own board production to cut even more costs.

circuit board

This presumably means their own GrblO (pronounce “garble-oh!”) board, And, perhaps more usefully for end users, the machine will ship ready to handle 80% lowers that do not have the rear pocket milled out, as well as the ones that do. As late as November 2014, they were still saying that only the lowers with the pockets milled out would work.

Ghostgunner test

Unlike every other technology firm, they can’t put their software or firmware on the web, and they can’t even post their user manual.They have run into difficulties, not unexpectedly, with the various Fed agencies that are supposed to license technology “exports,” which theFed defines as having happened when you put software on the Internet.

At least one GhostGunner did make it into private hands, in Texas, where it showed up at a demonstration at the state Capital. has a full report, complete with video, but we thought these quotes from Cody Wilson show that he’s playing a somewhat different game than your average entrepreneur:

I thought CATI [Come And Take It Texas] demonstrated the machine, and presented themselves, like men truly jealous of their Liberty.

As for my own objectives, if I can’t get you to stop asking permission from your Government, I can at least demonstrate its overcoming at its front steps.

Food for thought, that.

Personally, we just think a programmable, highly-rigid, easily fixturable and open-source CNC endmill has a lot of uses around here. Our revolution is a technological, not political, one. But freedom is the greatest and yet, the least harmful, of intoxicants, is it not?

“The Gun is its Own Tool Kit.” — Browning ANM2

This is Your Gun pG1One sign of a gun design that is not completely thought through is a requirement for special tools for disassembly. These days, most guns are designed for disassembly without any kind of offboard tools. But this was not always the case. John M. Browning was one of the first designers to consistently design guns to be disassembled without anything special. And it was a bit of a marvel, as the tone of this excerpt from a naval aerial gunner’s manual called This is Your Gun reveals:


In an emergency, the gun can be stripped with nothing but its own parts as tools. Use the point of a cartridge or the cocking lever pin to depress the oil buffer body spring lock.

Key parts in the oil buffer assembly:

Key parts in the oil buffer assembly: Oil Buffer Body Spring Lock (14a); Accelerator Pin (13), Accelerator (12). Oil Buffer Tube Lock (11)

Use the cocking lever pin to drift out the sear stop pin and accelerator pin.

Many of the parts mentioned are in the Bolt Group. In the order that they're mentioned:

Many of the parts mentioned are in the Bolt Group. In the order that they’re mentioned:  The Cocking Lever Pin is Nº 7. The Sear Stop with the Sear Stop Pin is Nº 8. The Cocking Lever is Nº 6.


 Use the flat tip of the cocking lever as you would use a screw driver to remove and replace the sear stop, oil buffer tube lock, the cover latch spring, and cover extractor spring. Use the oil buffer tube lock to pry the handle of the trigger bar pin out of its hole in the side of the receiver.

Use the sear stop pin to drift out the belt feed pawl pin.

The Belt Feed Pawl Pin is #5 in this illustration. US Navy.

The Belt Feed Pawl Pin is #5 in this illustration. Cover Latch Spring is #8, Cover Extractor Spring #9. US Navy.

But use these methods only when absolutely necessary and take care not to damage the parts used as tools. Never use the driving spring rod assembly as a tool.

Conversely, having a gun like this that can be disassembled and reassembled in field conditions without a bench full of tools is a marker of good design. This kind of design is more commonly encountered now than it was in Browning’s day, which speaks for Browning’s lasting positive impact on firearms design.

Mike Pannone: Making an M4 Run like a Gazelle

This article has been around for years, but it’s still worth reading. Mike Pannone is a fellow 18B and someone with nearly immeasurable M4 experience. He was an instructor for, and one of the designers of, AWG’s combat shooting school, which prepared a lot of guys for successful combat in Iraq and Afghanistan. A friend who was the SJA at AWG raved about that course. Mike is pretty well known in the shooting and training community.

Worked for us.

Worked for us.

Mike has very extensive comments on the M4 at Defense Review, which stem initially from a discussion of fouling. We’ll just quote his conclusions from this piece below, and also recommend his article on reliability issues, and his follow-up on diagnosing the root cause. Conclusions from what we suppose you could call the “fouling piece“:

Fouling in the M4 is not the problem. The problem is weak springs (buffer and extractor), as well as light buffer weights (H vs. H2 or H3). With the abovementioned drop-in parts, the M4 is as reliable as any weapon I have ever fired, and I have fired probably every military-issue assault rifle fielded worldwide in the last 60 years as a Special Forces Weapons Sergeant (18B). An additional benefit of the heavier spring/weight combo is that it transmits the energy impulse of the firing cycle to the shoulder over a longer duration, lowering the amount of foot pounds per second and dramatically reducing the perceived recoil. Follow-on shots are easier to make effectively, and much faster, especially at 50 meters and beyond.

I reliably fired 2400 rounds (80 magazines) on a bone dry gun, and I would bet that is a lot more than any soldier or other armed professional will ever come close to firing without any lubrication whatsoever. So, disregard the fouling myth and install a better buffer spring, H2 buffer, enhanced extractor spring and a Crane O-ring (all end user drop-in parts). With normal (read “not excessive”) lubrication and maintenance, properly-built AR-15/M4 type rifles with carbine gas systems will astound you with their reliability and shootability.

via The Big M4 Myth: ‘Fouling caused by the direct impingement gas system makes the M4/M4A1 Carbine unreliable.

There is a great deal more to the article than that; we just gave you a little bit. (For example, if you read the whole thing, he provides the sources for the upgraded parts he uses).

Mike’s articles are collected at the CTT Solutions website, although the articles link back to DR.

From the Academy to the Arsenal (long)

secret1The weapon was developed in the greatest of secrecy. It was born in a physics lab before the war, but during the war became a massive project: led by physicists; employing tens of thousands on detailed tasks whose application they did not know; secured by barriers, unsmiling military police, security clearances and teams of counterspies; and encompassing a wide range of industrial effort. It was shared under the most stringent security guidelines with Britain alone, and gave the nations of the Anglosphere combat power unimagined before the war.

Its developers, led by a man whom they all came to admire enormously, were a cross-section, not of society, but of the academy: more diverse ethnically than American society at large, scarily smart, and sometimes, endearingly (or irritatingly) eccentric.

To take it from the blackboards of physicists to the Arsenal of Democracy, new precision needed to be achieved in old technology, new technology entirely needed to be invented, and the frontiers of miniaturization pushed far beyond the 1941 state of the art. The weapon was useless if it could not be delivered to the close proximity of its intended target, so it had to be shrunk, shrunk, shrunk from the initial conceptual models, which would never have fit inside the delivery system.

It required new methods of testing and evaluation to be developed, to prove that it really would work when put to the test. Once it passed these tests, its effect on the enemy was devastating.

Exposing it became a major objective of Soviet spies and the traitors who had sold themselves to them, including some who would be caught and punished, like Julius and Ethel Rosenberg, and some who were too highly placed for suspicion to stick, like Lend-Lease head Harry Hopkins, who tried and failed to have samples sent to his real superiors where his true loyalties lay: the Soviet Union.

We’re referring not to the hoary old story of the Manhattan Project, but the equally old, but much less-known, tale of the proximity fuze, or as its WWII cover name called it, the Variable Time, VT, fuze. That cover name has stuck and prox fuzes are still often called VT, despite the fact that it was intended as a deliberate obfuscation of what the fuze really did: solve an “impossible” gunnery problem of the 20th Century.

MK53 proximity fuze

The Problem Was Hitting Moving Aircraft

Now, if you’ve ever cocked a cannon in Army or Marine artillery, you know a bit about the uses of VT; we’d like to ask all of you to be quiet, we’ll get to those shortly. But they’re not why VT was invented. The problem was anti-aircraft gunnery. In 1941 every military in the world was shooting at aircraft or preparing to do so, and they were missing. Just about all the time.

Airplanes don’t just sit still and let you whack them with your 88, you see. They move around. AA guns had compensating sights, and gun batteries had fire-direction computers, that could calculate where the airplane would be when the shell got to it — more or less.

(These computers were analog computers, with gears and knobs and whiz-wheels instead of circuits and programs, but they were faster and more reliable than the first many generations of digital technology that would replace them a few decades hence).

There was a little imprecision (not much) in the computer. There was a little more (again, not much) in the laying of the gun. Naval dual purpose and AA, and Army anti-aircraft guns, of the period were extremely accurate, but the problem still remained that for every shell delivered adequately to a vital structure of a Heinkel 111 or Aichi D3A, a much larger quantity, to steal a line from Maxwell Smart, “missed it by that much.”

A miss was as good as a mile, as the saying of the period went. In the initial effort to make a near miss somewhat more hazardous to enemy aircraft, various sorts of fuzes were conceived. The most common was a simple time fuze, and a lot of effort went into arranging the fire direction computer to support the optimum setting of an air defense battery’s time fuzes for effect on target. (Terrestrial artillery tried to use time fuzes to secure airbursts against troops in trenches, foxholes, or open-topped bunkers, too). An antiaircraft alternative was a barometric fuze, detonating at a preset altitude. This required fuze-setters to know the altitude of enemy aircraft (leading to the development of height finding radars, but also leading to the use of airplanes to shadow attackers and report their altitude to defenses). It also required them to know or have a way to dial in to the computer the ambient air pressure.

Winston Churchill describes this problem in Volume 2 of his History of the Second World War, (p.395), and notes: “[A]n aeroplane end on is a difficult target and a contact fuze will work only on impact.” Britain was working to develop such a fuze in the Battle of Britain, and while they were able to bring a mockup into the Cabinet Room to show Churchill, miniaturizing it was proving difficult, maybe impossible. So the British were stuck with the state of the art: impact, time and pressure fuzes.

These fuzes, the state of the art at war’s outbreak, would also be as far as the state of the art of Imperial Japan or Nazi Germany ever got. Many thousands of them had to be fired to ensure a hit on bombing aircraft. The watchword of the bomber theorists was, from the days of Douhet, “The bomber will always get through.” And it’s not easy to find an example of an aerial attack turned by time or baro fuzes alone, even in the sanguinary early days of RAF Bomber Command: the bombers took some hits, but most of them came through, unless they were hit by fighters, too.

How the VT Worked

The VT is essentially a small radar inside an artillery fuze. Initial concepts were wide open: photoelectric, acoustic, active radio (radar principle), or passive radio (detecting aircraft engine ignitions). In the end, after some experiments with photoelectric fuzes, active radio was chosen. Teh fuze radiated a radio wave, and if it received a reflection back, which it would if it were close to a target, pow! Here’s how E.D. MacAlister explained it to scientist Ralph B. Baldwin, who was just joining the project in 1941:

It’s really simple in theory but extremely difficult to convert into practice. The fuze is simply a specialized radio set. There’s a battery whose electrical energy is released by setback, the shock of firing from a gun. This battery furnishes three different electric voltages: one for the filaments of the vacuum tubes, one for the plates, and one for the grids. One of the tubes is an oscillator. In the nose of the fuze is a metallic cap, which together with the rest of the shell acts like a dipole. The oscillator tube thus has an antenna and emits a high-frequency radio wave in particular directions from the shell.

This continuous radiowave surrounds the moving shell, and when the shell pass is close to a target, the letter reflects a small amount of radio wave energy back to the fuse where it is detected by the same to been sent out the wave in the first place.

prox fuze exploded view

The plate voltage is “modulated” by the reflected wave, which is at slightly different frequency than the outgoing wave due to the relative motion of the shell and target. Thus, a beat note is set up and the plate voltage varies in frequency within the audio range of a few to a few thousand cycles per second. This audio frequency voltage variation is then passed through a three-tube amplifier.

When the period of the audio frequency wave and also its amplitude or intensity are exactly right, a thyratron tube, serving as a switch, is discharged. It completes a circuit that releases an electrical charge, which meanwhile has been stored in a condenser. The surge of electricity goes through a tiny wire in an electric squib, much like a dynamite cap.

The wire gets hot and the explosive in the squib goes off. This tiny explosion sets off about a cubic inch of a sensitive explosive called tetryl in the auxiliary detonator (auxdet to the Navy, booster to the army), which is at the bottom of the fuze.

This explosion sets off the explosive loading of the shell and it bursts the steel shell body into many hundreds of high velocity fragments. From the time the fuze says ago, the shell travels less than one foot before it bursts.

Now, if that sounds tough to follow, relax: even Baldwin found it “pretty heavy going.” But we include it here as (1) an illustration of the complexity of this pre-transistor electronic innovation, and (2), because we can’t imagine a simpler or clearer technical explanation of the fuze’s working than McAlister’s.

How the VT Was Developed

It started as an academic exercise, and began at Columbia University, but later found a permanent home at the Johns Hopkins University Applied Physics Lab in Baltimore, which was established (in part, to accomplish this task) in 1942. While the Manhattan Project was full of brilliance, the guy at the top of the APL and therefore the APL’s proximity fuze effort was Norwegian-American scientist Dr. Merle A. Tuve, the APL’s founder.

First they had to decide how to do it. They worked in parallel on optical and radio fuzes at first. It soon became apparent that the radio-frequency fuze was a success, and optical fuze development was cut off. (Unlike radio fuzes, optical fuzes worked only in daylight conditions).

The developers had to demonstrate the individual parts of the system; then, miniaturize them; then ruggedize them. From starting work circa 1940, Tuve had a working system within a couple of years. Decades later, the members of the team remembered the day when one of the proximity fuzes, fired from a Navy 5″ gun out over the water, worked for the first time. At the time, nobody made a note of the date! But the occasion was never forgotten. That day, only one of several test shots worked, the others detonating early or plowing into the water without detonating at all. But that one shot proved that the system was feasible. It had passed from science into the realm of engineering somewhere along the way.

British tube type VT fuze

This is all the more remarkable when you consider that the transistor, and all semiconductors, were over 15 years in the future: the fuze would have to work with the then-known electronic parts: vacuum tubes, capacitors, diodes and resistors, all powered by batteries. All these parts had to be ruggedized to survive the harsh environment of a cannon shell, with 20,000-g acceleration, 25,000-RPM rotation, and wildly varying temperatures all part of the fuze’s eventful last few seconds of existence.

The hardest parts to ruggedize were the batteries, which you might not expect, and tubes, which stands to reason. Their flimsy wires and glass enclosures were not optimum for high acceleration forces. When the scientists had tubes with 90% reliability, they still couldn’t relax: since the system needed three tubes to work (oscillator, amplifier, and thyratron), 90% reliability of any individual tube meant  fuze that wouldn’t work even three-quarters of the time, and that even if every other component was 100% reliable. (The three tubes would have to be 96.6% reliable for the tubes to be overall 90% reliable, again assuming 100% function of everything else).

The lab aimed for an overall 80% reliability of fuzes in the field, which they came to learn meant they could accept only a very low failure rate of the tubes. Even an 80% reliable VT fuze was a godsend to the Navy.

Final and near-final fuzes were tested against mockups of Japanese and German aircraft, and against drones. In April 1942, an unmanned Piper Cub was hung from a balloon hundreds of feet over the water at Parris Island and shot at with live fuzes, but explosive charges replaced by a black-powder marking charge that would make a visible puff of smoke, but not fragment the shell. 20% of the shots would have been hits.

On 12 August 1942, a drone imitating a torpedo bomber “attacked” the new cruiser, USS Cleveland, in a live-fire test. A first drone failed before coming into range. The second bore in on the ship’s beam. Cleveland’s 5″/38 battery shredded the drone, taking it down with under ten rounds fired. A second drone was shot down just as quickly. The drones were the usual anti-aircraft targets, and they never got shot down. The drone officers had to report to the test officer that they were out of targets. The next day, they had located one more, which was set to emulate a level bomber, and attacked fruitlessly with time fuzes. The battery loaded the VT fuzes and destroyed the drone. End of test.

The crew of Cleveland was counting on one more liberty before joining the war, but they’d just seen the successful test of the Navy’s latest secret weapon. The Navy ordered the ship to drop off the APL scientists and technicians, but to keep sailing for the Pacific. The fuzes, already in pilot production, were suddenly a hot item in Naval supply channels.

The Fuze Goes to War

By October 1942, production was up to 500 fuzes a day and they were being flown from the factories to West Coast and Pacific depots. Production ultimately involved five companies performing final-assembly duties, with components coming in from over 100 factories belonging to some 87 businesses. Most of the component makers didn’t know what they were making parts for.

This fuze setter was part of the equipment in a 5" dual-purpose mount.

This fuze setter was part of the equipment in a 5″ dual-purpose mount. The fuze setting was directed from the fire director down to the secondary battery plotting room down to the mount. Wartime secondary armament instructions (.pdf) from the USS Massachusetts (battleship).. 

On 5 January 1943, off Guadalcanal, the first Japanese airplane ran afoul of VT fuzes fired by the aft 5″ battery of the light cruiser USS Helena. Only three salvos were required to down the Aichi D3A “Val” dive bomberThe carriers Enterprise and Saratoga were also equipped with the new technology at that point, and from that point on the Japanese began having a harder time sinking American ships.

(Aside: Helena, CL-50, was famous for her gunnery, which would lead to her sinking. Japanese destroyers used her muzzle flashes to target her in July, 1943 and slammed three torpedoes into her, causing the ship to break into three pieces and sink with a loss of over 180 lives. Most of the crew did survive, although some were not rescued for days. A Helena sailor’s remains were found as late as 2006 on one of the straits islands).

Along with the fuzes, the Navy hit up the APL for new gun directors that would be optimized for VT, not time, fuzes. These went to sea for the first time with the battleship USS Missouri, and were effective against kamikaze attackers.


The VT Fuze Joins the Army

The highest priorities for fuze development were Naval, and so the Mk 32 was the first made, soon followed by equivalents for Royal Navy applications. But the VT Fuze was developed for Army anti-aircraft applications almost as soon as the Navy had it in production; one of the Cleveland’s drones was killed with an early prototype of the Army 90mm AA fuze screwed via adapter into a Navy 5″ shell. The Army fuze came from miniaturization developments that led to the Mk 45, and the British soon got forces for their Army AA guns as well (just in time for the V-1 buzz bomb attacks). But the AA application was only one application for proximity fuzes.

The more fruitful application, and one that would be key in several late-1944 battles, was creating a guaranteed airburst at an optimum height over enemy troop positions. The munition was, in a word, murderous to troops that didn’t have substantial overhead cover. LTG George S. Patton gleefully reported on the devastation wrought on German troops caught in the open by VT-fuzed barrages during the Battle of the Bulge.

At first, the VT fuzes were such secrets that they were removed from artillery positions during visits of foreign VIPs or American or British reporters. By war’s end, the effect of the fuze had let the cat out of the bag.

How the VT Fuze Spread to Other Nations

The Germans had heard rumors of the fuze, but never got hold of a working copy; they thought they were dealing with a “fuze with an electric eye,” and they tasked their spies to find out. The spies (Frederick Duquesne, Herman Land, and Lilly Stein of the 33-member Duquesne spy ring) never found the detailed information for the still-under-development fuze before a double agent and an FBI sting led to them being rolled up in 1941. The picture shows Duquesne, right, a retread WWI saboteur, with double agent William Sebold, left, a naturalized American who had immediately gone to the FBI when a Dr. Renken of the Abwehr (real name, Nickolaus Ritter) approached him in 1939, and was taken by FBI agents through a two-way mirror (note the clever positioning of the calendar so as to be in frame). The Deutsche Wehrmacht never got the secrets of the American fuze. Captured documents showed that the Germans had been trying to develop such a fuze throughout the war, but had made less progress by 1945 than the APL made by 1942.

The US shared the VT fuze with Britain from the beginning of development. After the initial development for the 5″/38 dual-purpose naval gun, and concurrent with development for additional US AA weapons, 4 Royal Navy calibers were added to the development schedule. Later, after the development of land fuzes for the US Army, 6 more for the British Army were added. By war’s end,  VT fuzes had been adapted to 28 different shells. The British had developed their own conceptual fuze as early as 1940, but they’d gotten stuck on miniaturizing and hardening the electronics, and welcomed the US project.

The Japanese never understood why they launched so many kamikaze Special Attack aircraft to achieve such meager results. With a few well-known exceptions where attacks saturated US defenses, the entire Special Attack program was a waste of resources and of men’s lives, and prox fuzes are one reason why. Of course, Japan was beaten by the spring of ’44, but wouldn’t believe it until they’d suffered the most absolute naval defeat since ancient history: while the Navy’s had some 2,500 vessels, most of them were small craft; all capital ships were on the seafloor, and the largest ship left fully combat-worthy was a cruiser.

The Russians got wind of the fuze and tried to come by it legitimately, first, by requesting it from Lend-Lease. They were turned down. Then, they tried to put pressure on through Harry Hopkins, to bend the rules (Hopkins frequently did this for his Soviet masters) and that didn’t work, either, because the subordinate officers wanted it in writing. Meanwhile, they tasked espionage assets, and the couple that came through for them with the VT fuze design would go on to fame — Julius and Ethel Rosenberg. Well, fame and Old Sparky at Sing Sing.

And there’s irony for you: far more Americans have been killed by Soviet copies and improvements of American VT fuzes than have been killed by Soviet copies and improvements of American nukes. But the spies who gave up both weren’t even charged for the little, seemingly inconsequential, war-winning little weapon. If they had been, they would never have received the death penalty. But if you truly understand the weapon, they deserved to fry for this as much as anything.


Baldwin, Ralph B. The Deadly Fuze: Secret Weapon of World War II. San Rafael, CA: Presidio Press, 1980.

US Navy. Radio Proximity (VT) Fuzes. Naval Historical Center Website. Retrieved from:

More Bullshit from our Favorite Lobbyist

Yes, it’s Major General Scales again, last seen blaming “jammed M4s” for the deaths of 9 guys whose valiant deaths we recounted in our two part Wanat series (Part 1) (Part 2), absolving the M4 in the process. Tam in the comments steered us to more of his unique brand of wisdom, from October, 2013. She asks, “[D]id you see this eulogy-turned-shopping-list from Scales?”

Scales explains that all the Army really needs, to prevent the kind of desperate fights that produce Medals of Honor, is a few simple trinkets, gimmicks, and imaginary technologies to be produced by his defense-industry clients. We’re not making that up! Scales:

[CPT William Swenson] was the sixth soldier or Marine to receive the medal for heroism in Afghanistan. All six stories are remarkably similar in that none of these incredibly brave men should have been in a position to have earned the medal. Had soldiers in these engagements been adequately provided with a few cheap technologies perhaps they might have avoided the bloody traps that precipitated their heroic actions.

Uh-huh. “Buy the right stuff, and you no longer need people with The Right Stuff.” You can see where a Macnamara-era officer, who fled fast and far from troop leadership towards academic pursuits, might come up with that. So what are these specifics, that will render valor superfluous in our all-conquering robot army of the future?

  • “Cell phones” instead of “bulky radios.”

But did you happen to notice in the video the bulky radio stuffed in Swenson’s backpack? This battle was fought in 2009 a time when rag pickers in Mumbai had cell phones. Why can’t our fighting men and women have cell phones in combat?

"Sorry, Mom. I guess I butt-dialed!"

“Sorry, Mom. I guess I butt-dialed!”

  • Helmet cams!

Imagine for a moment that Swenson, like the medevac crewman who took the video of Swenson, had a simple camera on his helmet capable of displaying the ground situation and linked it to screens in the Operations Center. Had the officers in the center seen the action in real time though Swenson’s eyes perhaps supporting fires might have been immediately cleared long before Swenson was trapped in the kill zone. You can buy helmet cams at Walmart.

"Imagine all the peo-ple... livin' life in pee-ee-aa-a-ee-ce."

“Imagine all the peo-ple… livin’ life in pee-ee-aa-a-ee-ce.”

  • Moar Dronez!

What if our military had been able to deploy enough drones to put a set of aerial eyes over every ground patrol marching into a dangerous and uncertain situation? Surely had a drone been overhead the Taliban would never have dared to open fire.

  • People Sniffers!

[W]hat if the one of the lead element carried a sensor that detected movement or the metabolic presence of humans nearby? Such devices are easy to develop and the technology has been in use by civilian security companies for years. Again, had Swenson’s team been warned there would have been no ambush and no medal.

People Sniffers: Yesterday's Bad Idea, Today.

People Sniffers: Yesterday’s Bad Idea, Today.

  • The M25 surviving half of the OICW boondoggle (emphasis ours)

[T]he M25 “smart grenade launcher” … uses a laser beam to program a grenade to explode over the heads of the enemy hiding behind protective cover. Such a weapon in the hands of Swenson’s team would have taken out the Taliban with ease. After a decade of development the Army hopes to have the M25 in the hands of troops this year…maybe.


  • A lightweight heavy mortar!

What if Swanson [sic] had had access to a really good “carry along” heavy mortar? What if the mortar bomb had precision GPS guidance such that the first round landed directly on the Taliban? With such a weapon Swenson’s fight would have lasted about three minutes instead of nine hours.

remco long range mortar

Drat! The hard part is getting the Taliban into the included Exploding Pillbox.


We’ll get to his conclusion after we deal with these individual beefs, but as you see the essence of Scales is that everything the military has is crap, so they need to bow down before his brilliance (and not incidentally, make his defense-contractor meal ticket cash-registers ring).

So what’s wrong with the idea of….

…Combat Cell Phones?

Here’s what Swenson’s “bulky” radio could do that the retired General’s Samsung can’t:

  1. Use high-level, keystream encryption. This is kind of a big deal. Officers of Scales’s era, who don’t recognize the enemy’s initiative and seriousness, were always a problem with radios, because they could never be bothered with encryption. Yes, the Taliban and its allies do employ signals intelligence against the US and Coalition.
  2. Work with limited and even no on-the-ground infrastructure. See, a cell phone needs… a cell tower. You can’t count on those forward of friendly lines.
  3. Meet military demands for ruggedness. We’ve had military radios fall 250 feet due to a lowering-line failure on a parachute jump, and survive vehicle and aircraft mishaps. They can get wet (true, in Scales’s day, you had to put a baggie around the handsets), get hot, get cold, and the stout little beggars keep working. Anybody want to see our collection of dead iPhones?

…Helmet Cams?

That’s just what we need, a way for deskbound leaders and other rear-area drones (the human kind) to kibitz on combat. Call of Duty 5, Pentagon Edition? Does anybody remember what happened when one of the Army’s Unique and Special Snowflake™ intelligence analyst privates decided that he could interpret some gunship video?

Already, it’s a huge problem being able to fight your unit without constant demands for updates from self-important gawkers at levels and levels of higher headquarters. The sort of Type A personalities that become colonels and generals can’t resist the temptation to try to direct their younger analogues who are fighting in real time. There is a fine line, perhaps, between assistance and micromanagement. But Army culture (at echelons above combat, anyway) lionizes the micromanager and we’ve seen very few higher-echelon leaders who failed to stomp over that line with both big feet.

Then, there’s the bandwidth problem. The US military uses vast quantities of bandwidth, the majority of it for nonessential purposes. But imagine what happens when we start streaming helmet cam from everybody on patrol to the vast majority of everybodys who never go on patrol.

But that’s OK. In Scales’s world, sparkly unicorns will poop the bandwidth we need to flow all that video by satellite. Maybe he can also declare an end to communications latency whilst using satellites!

…Moar Dronez!?

This is a great one. Because the Army alone has had literally dozens of drone-development programs, distinct from those at the Air Force, the other services, Joint programs, and other government agencies, all of whom went all-in for drones after their utility was proven in 2001-02. (Back then, everybody in Afghanistan had to share 1-3 Predator flights a day). The Army’s boffins had wanted UAVs for intelligence collection long before the war. (Here’s a staff college paper (.pdf) on requirements from 1990. And yes, the joint programs described in then-MAJ Harshman’s study lost control of service UAV requirements during the war).

There are some problems with drones. They’re not, as Scales seems to imagine military technology to be, FM (that’s an old radioman’s joke: F’n Magic). People have to operate them and interpret the product of their sensors. For instance, at the time of the Swenson battle, there was a small drone, the Raven… but it would take two men out of the fight, one to program the drone’s flight, one to operate its sensors.

But the bottom line is that drones can’t replace people on the ground, and they can’t be everywhere people on the ground go; they can’t operate in crummy weather, unlike, say, infantrymen, who will cheerfully tell you that they seem to operate only in crummy weather. And for all the spending on drone development, very little of it filters through Scales’s defense contractor pals and makes it down to the war fighters.

…People Sniffers?

Let’s just re-repeat (threepeat?) some of Scales’s discussion on this,

But what if the one of the lead element carried a sensor that detected movement or the metabolic presence of humans nearby? Such devices are easy to develop and the technology has been in use by civilian security companies for years.

Because fighting a war among the population, you can just bring your weapons to bear on any hint of movement, or the waft of human pheromones, with your eyes closed. Scales is showing his “once they’re dead, they’re all VC” heritage here. But he’s also showing a remarkable ignorance of the technical history of the People Sniffer (.pdf), Projects Muscle Shoals (.pdf, in-progress whitewash), Igloo White, and all those Macnamara Line developments. Those things were all costly failures.

You know how we actually got actionable intel off the Ho Chi Minh trail? We put human eyes on it, and yes, the guys in that project got shot to $#!+ a lot and wound up with more than their “fair share” of MOHs.

…the M25 boondoggle?

Can you say SPIW? It was the Weapon of the Future® in 1960, and it still is…. There are several problems with the M25, but they basically come down to this: it’s optimized to meet a requirement that doesn’t occur all that often in combat, and that can be answered better by a well-trained 60mm mortar gunner and a lot of rounds. In other words, even if it worked 100% (which should occasion great mirth among those who worked with it), it is still inferior for its special purpose to a common general-purpose weapon already in the inventory.

But new space-age grenade launchicators are sexy and get written up in tech magazines (as well as, grease the big DOD prime contractors and their lobbyists). More mortar ammo for training is distinctly unsexy, and benefits only some dirty, uncouth infantryman, not a K Street lobbyist in a $3k suit. Which brings us to one of Scales’s weirdest demands:

…the paradoxical light heavy mortar.

… a really good “carry along” heavy mortar?

You mean like the 60 (which actually comes with a sling and can be carried with a round in place and fired with a trigger), and the 81s that some grunt units have hauled on patrols? Or a 120mm where the ammo is too heavy to carry more rounds than you need to set the baseplate?

Scales can be excused for not paying attention to mortars and understanding their current state of development, but the size of the mortar has more to do with its range than its lethality. And ammunition improvements have been remarkable over the last few decades. That light 60mm mortar is handier than the World War II vintage 60 despite its greater length, is hell for accurate, and has the range and lethality of the 1960-or-so vintage 81mm mortar.

And then, the problem is not the weight of the mortar, but the weight of the ammo. A round for the 60 weighs 2.5 to 4 lbs, a round for the 81 9 to 10 lbs.

And here’s a fun fact, that infantrymen all know but that may not have penetrated to the ranks of Retired War College Panjandrums: the M224 60mm mortar greatly outranges the small arms that engaged CPT Swenson’s advisory team, or any other small arm, for that matter. Its accurate range is over 3500 m on max charge, nearly 4,000 yards. 

As far as precision-guidance goes, in a direct-fire situation, a decent mortar gunner who has had ammunition to practice with and develop his skills is utterly deadly with a 60. The best precision-guidance computer on the planet, at least for this purpose, is attached to the relevant sensors by a pair of optic nerves, and located in the brain-case of an infantryman who has been given challenging training, multiple targets, a case of mortar rounds, and some friendly competition.

But, there’s nothing in there for K Street. Or technology magazines. Sorry about that.

Some General Comments

Some of these things amount to, “Gee whiz, maybe if you grease my clients they can repeal Newton’s Laws of Motion.” It’s an example of Full Retard, defense intellectual division. And yet Scales says that it’s he who’s against wasteful defense spending:

These and other soldier-saving technologies could have been developed and fielded cheaply and quickly years ago.

Gee, does he mean the billions blown on the Macnamara Line and its sensors in the 1960s and 70s, cheaply, or the billions blown on drones, cheaply, since the 1990s? (Well, drones go back to the Kettering Bug if you want to get all inclusive).

Yet, after ten years of war the ground services, the Army and Marine Corps, remain starved for new, cutting edge life-saving materiel

Bullshit, bullshit, bullshit. We call bullshit (threedundantly). The ground services have seen a vast quantity of high-quality, highly-useful, technologically improved equipment. The trooper of 2014 carries stuff that was fuzzy theory in 2001. We were there during halting, experimental and limited deployments of drones, blue-force trackers, and sophisticated counter-IED technologies, to name a few. We were there to see the capabilities of our electronic intelligence collector attached elements take off. We were there to see contact teams, and PEO Soldier, and tech reps from contractors, and they weren’t bringing all the goodies just to us in the white SOF, or our bros on the darkside, or the always high-tech Air Force, but plenty of things that eased the rocky path of the rifleman, good old 11B or 0311.

Want to see life-saving material? Shake out the M3 aid bag a retiring SF medic threw on his garage shelf in 2000, and then shake out one of today’s medics’ kir. And never mid the stuff, the training of the people is the biggest lifesaver going, and has undergone a quiet revolution. We lost guys in 2001 and 2002 to tension pneumothorax and exsanguination, and those are hardly the killers now that they were then.

[W]hile the Department of Defense and their big defense company allies continue to spend generously on profitable big ticket programs like planes, ships, missiles and computers. Soldiers’ stuff is more Popular Mechanics than star wars. But Captain Swenson and his six Medal of Honor colleagues might have had a better day had the nation spent a bit more to give them a technological edge over the enemy.

What they haven’t got is the kind of armchair brainstorms that Scales launches in their general direction, but maybe that’s because that’s not what they’re asking for.

What we need is a human edge over the enemy, not a technological one. Technology is a force enabler and, in very rare best-cases, a force multiplier. As a rule of thumb, a dollar spent on training beats a dollar spent on stuff. But what is the first thing a retrenching Army cuts?

And this confused fellow was a Major General and an Army War College Imperial Wizard or Exalted Octopus or whatever they call it. Lord love a duck. If this is the kind of talent we cultivate at that level, no wonder we haven’t won one clean since V-J Day.

PPShooting Around Corners

Waffen Revue 25 - StG44If you’re the kind of gun and history geek (hey! own it) we generally attract to the blog, you’re already familiar with the Krummlauf (“crooked barrel”) attachment to the German MP.44 series assault rifles.

The Krummlauf  is well-documented in books like Small Arms of the World, surviving period documents, and that sort of thing. It was made in several versions, differing in the degree of “bend” (30, 45, and 60º IIRC) and could be used for firing from cover (down), or the whole weapon could be turned over for firing around corners (sideways). It had its own 1.5x optic, and the extended, curved barrel was both vented for relief, and rifled.

Whether it was intended for urban warfare (firing around corners in the assault), armored warfare (firing from behind cover in a halftrack) or positional warfare (firing from trenches) is a matter of speculation. The problem with this kind of specialized weapon, for the Germans or anybody, is that you only need it once in a while, but you have to carry it all the time. That is, if you’re going to have any hope of having it with you on the rare occasion when you do need it.

There’s a number of surviving Krummlauf attachments and MP44 Krummlauf hosts, at least a half dozen, with at least two on the NFA registry (there are probably more that those numbers). One was auctioned recently by Rock Island and has a very complete description, with an explanatory video by Ian McCollum of Forgotten Weapons, on its auction page.

grease_gun_around_corners_ps_march_52The US experimented with something similar, but vastly simpler. We deleted the German prismatic sight, and didn’t even make a complete barrel, creating something more like a bullet trough for a spray of 230-grain solids to go off in the general direction of the enemy. This has been widely reported to have been done by the OSS. The historical writeups are thinner than on the Krummlauf, but they’re there. The gun seems to have first come to public attention in the Korean War era. For example, it was featured in Popular Science magazine in March, 1952 (image left). The article suggests that the gun was meant to be used, and hints that there might have been an optic, but, “Sights are secret.” The gun was also featured in LIFE in 1953, and those photos turn up online here and there.

LIFE OSS curved barrel

But we never knew until we stumbled over it on the excellent site, that there were at least two Russian variants of the same thing for the PPSh-41, which was made in staggering quantities. Unlike the common PPSh, these variations are extremely rare, probably for the same reasons of impracticality that limited distribution of the German and American ones. The more sophisticated showed a similar design approach to the Krummlauf, with the added benefit of being easily convertible in direction. This video shows the gun:


The second was a bent-down version, called in one reference the Model 1945, that looks more like a gimmick than a real, working gun.

ppsh-45 curved

It honestly looks like someone heated and bent a regular PPSh. (As we’ve seen from our recent M4 at Wanat series, heating barrels can be A Bad Thing®).


We’d love to have the whole who-shot-John on these, but we don’t. Maybe some commenters can help.

One of the most interesting questions is this: were the American and Russian “corner guns” simply examples of convergent evolution, or did they come about after examining German Krummlauf units?

Ian notes, in his video about the Krummlauf, that the Germans tried doing an open trough like the later American Grease Gun modification, but gave it up and went with a rifled curved bore instead.

The Burned Out Barrel Problem

As we saw over the last couple of days, it’s possible for a very brief period of very high intensity firing to drive a weapon to catastrophic failure. The managers of US small arms programs have identified two additional failure modes that are exacerbated by high rates of fire (and therefore, high temperatures: bolt failures and barrel failures).

Let Us Propose a  Way of Viewing Malfunctions

If you were to classify failures, you might measure them by the seriousness of their effect, or by its permanence. For example, a malfunction that renders the weapon unfireable (like a broken bolt, for instance) might be a Class A malfunction . A malfunction that degrades its performance in a militarily significant way (a burnt-out barrel, causing misses) might be a Class B malfunction. A malfunction that is relatively trivial (loose flash suppressor) in its impact on combat readiness is a Class C. Then, we’ll add a numeric value for where the repair must be made. We come up with a matrix like this:

Malfunction Matrix

Malfunction Repairability


Irreparable / Depot Org Repair Field Repair

Class A (total dysfunction)




Class B (serious degradation)




Class C (mild malfunction) C1 C2


The most significant, as in urgently addressable, problems, are in the upper left; the most trivial, the lower right. It might make sense to prioritize the Class A totals (in numeric priority) and the B1, irreparable serious problems.

This 2006 power-point from that year’s NDIA small arms meeting reviews a wide range of small arms program highlights, but we’re going to focus on two problems it identifies, one of them being an A1 or A2 problem (depending on whether your unit was authorized to stock the repair part or not): the failure of a rifle or carbine bolt. The next is a potential B1 problem: the shot-out barrel.

Bolt Failure in the M4

There are two common places where the bolt fails: in the web, where the bolt has had a large hole hollowed out for the carrier key, and having lugs simply shear off. The first of these is always a gun-down, non-repairable failure. Sometimes it can be detected ahead of time by carefully inspecting the bolt, under magnification.




In a grimy, operational gun these small cracks can go unseen. If one starts on one side of the web, it will soon crack through, and the asymmetrical stress is now loaded up on the other side, which is as battered and worn as the first one was when it failed — so it soon lets go, too. The bolt in the picture above would still function in the rifle — right up until the moment it didn’t:


If this malfunction happened in combat, the weapon in question would be reduced to the status of “bayonet handle” for the duration of the fight, and hardly anybody carries a bayonet any more. Big time Class A-1 failure, weapon down for the count, and you can’t fix it here. (If you are authorized individual repair parts at unit level, your organizational armorer can fix it. If not, you’re screwed, dude. This is why some savvy guys bring an illegal stash of privately purchased common failure parts on deployments).

Two other problems commonly seen on M4 bolts are sheared lugs and burnt-out gas rings. The weapon may continue firing, after a fashion, with these failures. But it’s a sick puppy and needs a trip to the gun vet, or these problems will worsen until it’s an A1 failure, too.

m4-m16_busted_bolt_lugThe failure mode of that lug is really interesting. You would think that the lugs would fail on an angle from where the forces bear on its after surface, and this one seems to have done that, at first glance. But look at the shear surfaces. The smooth part (usually where the failure started as a crack) is in the bolt pocket for the cartridge head. It’s possible that the stress that failed this bolt was the radial stress from an expanding case head, not the locking force applied to the after surface of the AR bolt.

If the first lug fails, the load which had perhaps been divided seven ways is now divided six. (We say “perhaps” because, without lapping the lugs in, there’s no guarantee you have optimum contact, and in fact, you almost certainly don’t in a factory gun, which is fine: there’s a margin in the design). So the force that sheared one lug that was one of seven bearing it is now laid on only 6 lugs… we can’t say for certainty when the next lug or lugs fail, but we can say it will be a shorter interval, in terms of round count, than it took for the first one to let go.

Hard use will damage a bolt within 3,000 to 6,000 rounds; cracks will be visible on inspection. Almost all M16/4s will show damage by 10,000 rounds. The damage may not be mission-stopping: what no one knows is how long a cracked bolt can soldier on like that.

Now look at the burned-out gas ring on a carbine bolt:



There should be three small gaps in the rings, and they should never be aligned, instead, always, staggered. This is a safety-of-operation item: these gas-check rings keep the combustion gases in the internal cylinder of the bolt carrier. It also produces hard-to-diagnose failures to eject, extract, and/or feed: you should always inspect and, if necessary, rearrange or replace, the gas-check rings any time you have the sort of malfunctions that might be due to weak strokes and short-stroking.

The estimated life of a carbine barrel closely tracks that of the bolt; from 4,000 to 6,000 rounds if used hard, 10,000 plus rounds if gently treated. The problem with barrel life is that it’s had to know when you’ve reached it. One way to judge it is empirical: your shot groups get larger and larger over time, because the throat erosion that is the primary cause of accuracy degradation is a progressive thing.


At first, given good aim, it’s not much of a factor, as the weapon has an accuracy reserve at most combat ranges. But soon the normal shot-to-shot dispersion and the increasing size of the shot group mean that even perfect aim is frequently unable to hit the target.



Now, here’s the kicker: the normal tool we use to measure erosion, the taper erosion gage, doesn’t work reliably. According to the presentation, it’s right six times out of ten, but the other four times it can fail either way, identifying a good barrel as a reject needing replacement (false positive) or failing to identify a bad one (false negative). The first error wastes a fortune scrapping viable barrels, and the second may send a soldier into combat with a weapon that will make him miss his enemy.

A tool this inaccurate is worse than no tool at all. We would do better to measure throat erosion by chucking the rifle in a machine rest and measuring the size of a five-round shot group, than to rely on the meretricious promise of that solid-seeming gage.

In addition to the throat-erosion problem, there is a secondary gas-port erosion problem. This manifests as many different symptoms: cycling problems, failures to feed and eject, changes in weapon cyclic rate, and degraded accuracy.

What ties these problems together?

Ever single one of them is caused or exacerbated by heavy use, especially at cyclic rates. The weapon will last much longer if it is treated with care and allowed to cool between shot strings. If it’s fired as if you’re faced with a human-wave attack from the 3rd Shock Horde (or if you’re faced with such an attack and need to fire it that way) it is ripe for any of a number of interesting and troublesome failure modalities.

So What’s the Answer?

In the maintenance world, you can replace “on condition,” by inspecting things to see if they’re still serviceable, and rejecting them when they fail inspection, or fail in service, or “on schedule,” replacing them on time. We all use these concepts every day: we replace our light bulbs when they burn out, but we change the oil in our car every 5,000 miles. That makes a certain sense; there’s no great consequences to letting a bulb go our, but if your motor oil fails to lubricate you’re looking at a big repair bill.

The Army’s approach to weapons historically has been to maintain them “on condition,” with operator, organizational, and depot-level Preventative Maintenance Checks and Services that are outlined in the maintenance Technical Manuals. But since those inspections don’t work, at least insofar as they want them to ID their failing parts with high accuracy, they’re trying to move to maintenance on schedule.

The proxy they’ll use for wear on the weapons will be round count, and the Army’s plan is to make a round counter a component of every weapon.

We wrote about this before, last April (looking at this same presentation, actually, but from a different angle):

The trouble is, of course, that logging rounds is a great deal of work. But if the whole Army could do it, we’d get a lot more information about how long small arms and their components are good for, and we could begin to schedule inspections and overhauls more intelligently. Too many inspections waste money, and some percentage of overhauls go and rebuild guns that don’t need it, while some other percentage of guns that need overhaul, based on their condition, don’t get picked up. (Army ordnance experts think that both of these numbers, the false positives and the false negatives, are about 40%).

You don’t have to wait for the Army to beat this problem; while automated round counters are in the future for most of us, some of them are coming online; and there’s really nothing wrong with the old-school approach of logging every round with a pencil and notebook. (That works fine for personal weapons. For issue ones, that may get swapped around a lot, it’s not so good).

Any and every weapon can be made to fail. The better its career is logged, the more likely the career will be long; the more operators (in the “users” sense, not “ninjas”) understand it, the more they will rely upon it. The better it’s understood, the more it can be improved.

And that all starts with a #2 pencil….


The SIG Brace / Not a Stock / ATF Letter Trip

donovan leitch 1967Remember the old Donovan song? Eh, unless you’re like us, old enough to remember the introduction of that new “dirt” stuff, maybe you don’t. The trippy 60s songwriter sang the very zen line:

First, there is a mountain, then there is no mountain, then there is.
First, there is a mountain, then there is no mountain, then there is.

To which we’ve always mumbled, “Don’t take the brown acid….” (Sorry, another cultural flashback). Anyway, Donovan’s flickering mountain is a bit like the various ATF letters explaining their attitude to arm braces on AR pistols over the last couple of years, since they first provided a Firearms Technology Branch blessing to the Sig Brace.


First, it was a stock that made the gun an SBR, then it wasn’t a stock, then it was.
Then, it wasn’t a stock that made the gun an SBR, then it was a stock, then it wasn’t.

We’re not sure what to make of the ATF apparently taking up the recreational herbs and spices of the Sunshine Superman his ownself, but we’ve been whipsawed by the letters and haven’t written about them. Regulatory stuff is kind of boring, at least until ATF shows up looking for someone to feed their stats machine and settles on you. (And trust us on this: every Federal law enforcement agency has a stats machine, and it looks just like the one in Fritz Lang’s Metropolis.)

Fortunately, the Prince Law Firm’s blog has been on it, and these guys are, like, real lawyers with bar cards, and ostentatious diplomas, and continuing education credits, and everything. Adam Kraut, Esq:

Well, it appears very clear that FTISB and ATF as a whole are paying very close attention to what people are doing and how they are utilizing products, including reviewing internet postings, pictures and videos. All of the stabilization/cheek enhancement products on the market have a legitimate purpose and have assumedly been approved by FTISB at some point. But, it appears that some individuals are not looking to purchase these products for their legitimate purpose and use and instead intentionally intend to misuse them from the moment they are purchased.

As was noticeably absent in the letter discussed in my blog post Cinderella and ATF’s Determination: The Fairy Tale of an AR Pistol to SBR through Magic, this letter does mention intent, in fact several times.

ATF didn’t appreciate people purchasing various stabilization products/cheek weld enhancements for the purpose of avoiding the payment of the NFA tax (which could constitute tax evasion). This is why the intent aspect, as stated in the definition, is important. If an individual purchases one of these products intending to use it in the manner for which it was made and then misuses it, as ATF previously held in the Bradley letter, he/she has done nothing illegal. There is no law dictating the end use of a product. However, if an individual purchases one of these products to install on their pistol and intends to use it as a faux stock, he/she has very clearly created an illegal SBR.

We think the consigliere has done a good a job as anyone can hope to of reading the ATF tea leaves, so we’ll leave it at that (do go Read The Whole Thing™).

Now, we’d like to make some comments about the ATF technology evaluation process in general. Kraut notices that they did something they usually don’t do, explicitly warn that this paper really isn’t worth more than the paper it’s printed on. He quotes commentary on the latest “brace” letter, this one to Thorsden Customs. What the letter itself (hosted at Prince Law) says, is:

In closing, we should remind you that the information found in correspondence from FTISB is intended only for use by the addressed individual or company with regard to a specific scenario described within that correspondence.

This is apparently new boilerplate. But the fact is, that is the nature of all ATF determinations. They are ephemeral, have no precedential value, and are only binding on citizens, not on the ATF. The ATF can, and does, overturn them at any time on nothing more than a whim, and the courts have rules that these will-o-the-wisp whims require near-absolute deference.

ATF-Molan Labe

Finally, a couple of exit thoughts: If the ATF didn’t take an elephant’s gestation to process SBR paperwork, maybe so many people wouldn’t be looking for an end-around. Want to increase compliance with the law? Make it easy and convenient. If somebody’s not making it easy and convenient, maybe they’re not really interested in increasing compliance with the law.