Category Archives: Weapons Technology

A Tale of Two Temperatures

Consider this graphic. It is a somewhat crude reproduction of one in the Rheinmetall weapons design handbook. Secretary of State John F. Kerry, who is unlucky at bicycles bur uncannily lucky with heiresses, thinks that we all should be criminals for discussing this online, so let’s all get our crime on and return to a subject we’ve discussed before, heat management  in automatic firearms.



The original of this graphic is a rather dull monochrome one in the style of the rather dull, unless you are the sort of gun geek that Secretary Kerry dreams of decanting into durance vile, Rheinmetall Handbook. Our copy is the German language version, because we read po-nyemetskiy, and wants $300-400 for an English copy, when there is one to be had, but has copies of this out of print classic for about €100. (Which is going to be lunch money if they keep letting Greece set continental fiscal policy). It took us several iterations to get the slopes about right, and we got the round-count wrong: it’s supposed to be two neat Teutonic bursts of 10, and we have a rather limited and non-Aryan 8 and 9, but with that difference noted, this graphic is  close enough to discuss the phenomena at issue. Here’s what Rheinmetall says about this graphic:

The barrel of an automatic weapon is, as a consequence of the normally high rate of fire, subject to extraordinary temperature demands.

[This illustration] shows the approximate course of temperature of the inner- and outer wall of an MG barrel in two bursts of fire of 10 shots each with a pause lying in between. Feel long, or many short, bursts of fire can drive the temperature of the inner wall so high, that it has a significant influence on the material toughness and therefore on its use and employment.

In this, the cadence of fire, the number of shots in a burst, the pauses, the lengths of the pauses, and the number of bursts of fire fired rapidly one after the other, in conjunction with the thermal resistance of the barrel material and the strength of the barrel walls all play a role.

Comparable barrel-life shot counts can therefore be reached with the same firing rhythm. Often the “French anti-aircraft rhythm” is used: this is 144 shots in 12 bursts of 12 shots each, with a 2-second pause after each burst and a 20 second pause after every four [bursts]. With MG barrels, a firing rhythm of 250 rounds in numerous bursts is often used.

For testing automatic weapons and their ammunition, Rheinmetall has developed an electronic Rate and Rhythm Control Apparatus, which is described below in section 7.7 [of this book].

Measures for increasing the life expectancy of barrels include:

  • Heat-resistant materials;
  • Chroming or Nitriding the interior wall;
  • Progressive twist and rifling profile in conjunction with barrel caliber tightening.

Less effective are cooling fins and water cooling.

Barrels for MGs and machine cannon must be rapidly interchangeable.

Now, this graphic is limited in its utility because in its original version, it comes without any numbers attached (accordingly, we have eliminated from the version we show you, the numbers we used for temperature (ºC; as you might expect the Handbook exclusively uses SI units) and seconds to approximate the original. But we can draw some conclusions based on the shape and gradient of the two lines.

Our take-away is that the key point is that the baseline is higher after each burst, and that the internal temps go higher in each successive round of each successive burst. What does not show on this line is the temperature where the barrel fails. As we have seen in the M4 experiments wherein a carbine was tested to destruction, this happens at a fairly predictable and repeatable, ergo constant from an engineering point of view, temperature. As Rheinmetall points out, several roads will get you to that temperature sooner or later.

Maybe he didn’t know this, but thus is why your sergeant whacked you upside the hemmet and told you to fire shorter bursts.

Looking at the Rheinmetall data, it seems that for their test weapon, whatever it was (MG3?), the barrel recovers its temperature rather speedily after the passage of each bullet momentarily superheats it. We would attribute this to the limited ability of a small projectile’s friction to heat the much greater mass of the barrel — this is also why the internal and external temps diverge so widely. But note that the internal temp continues to ride steadily as long as a steady sequence of pause and fire is applied, and at each pause the internal and external barrel temps have diverged more widely.

The implication is that an automatic weapons barrel is going to be heated to its limits at some point, moreover, at some predictable point, in any continuous fire regime, and while some of the magic designers have used over the last century, like chrome plating and stellite liners, can give you some more rate of fire on the margins, only changing out the barrel (not usually possible in a light automatic carbine) or otherwise giving the barrel a chance to rest and recover from high temps will prevent failure.

Thus endeth the lesson. Apart from further education which may come in the comments.


Geissele (ALG Defense) AK Trigger

Bill Geissele’s wife’s company, ALG Defense, makes products for more of a mass-market than the very sweet, fairly simple, Geissele AR-15 triggers that live in more than a few SOF M4s and Mk. 11/12/18s, etc. (Indeed, sometimes the Geisele triggers are authorized MFP 11 or unit purchases, and sometimes they are installed on a catch-me-F-me basis by unit weapons men or armorers). Along with the triggers for full-on M4s and HK416s, Geissele makes improved triggers in both single-stage and two-stage variants for a wide range of semi ARs. They’re not cheap, they’re not always in stock, but they’re good.

ALG Defense makes simpler AR triggers — and now, an AK trigger, imaginatively coded AKT. In this video Bill explains the objectives this trigger meets and talks about some of the challenges involved in designing it.

The AK, Bill says, “has a ton of sear engagement.” That’s what you, the shooter, perceive as the very long and very smooth takeup of the typical AK trigger.  (The SKS trigger has a similarly long, smooth engagement, suggesting that this may have been a standing Soviet / Russian design objective).

The result is an AK trigger that fits a variety of common receivers on domestic, imported, and kit-built AKs, and that reduces the trigger pull force and duration (including that all important very long sear dwell) significantly.

For example, Bill shows a graph of a stock AK trigger versus the ALG AKT; the stock trigger moves about 0.150″/4mm and takes about 4.5-5 lb. of pressure during that sear dwell period. The AKT takes up the slack more quickly and seems to come in about 0.065″ and just under 3.5 lb.

At about 8:30 he shows a 3D model (in Autodesk Inventor) of the trigger and walks through its function.

It fits some AKs with no fitting, but because of the wide variation in AK safeties, some AKs need a roll pin positioned so as to contact the safety. It’s explained in the video and in the AKT’s instructions.


Design: Feed Systems, or, From the Mag to the Chamber

One of the key components in the proper functioning of any repeating weapon — not just automatic and semi-automatic ones — is the feed system. The feed system comprises ammunition storage and all mechanisms that control the feed of a cartridge from that storage (named a magazine after the centuries-old military name for a storage place for powder and shot) to the firing chamber. Many, probably most, unreliable weapons are unreliable because of faults in the design or maintenance of their feed systems, and weapons that are known for high reliability, like AK rifles or Glock pistols, have had a great deal of attention to the feed system.

The first successful feed systems were very different from what is commonplace today: you had the cylinder of revolvers, gravity-feed systems on early manual machine guns, the Accles feed of the later models of Gatling gun, all of which except the revolver were evolutionary dead ends and exist only in museums and collections.

Conventional Wisdom

Rejecting unreliable and complex feed systems in favor of more reliable and simpler ones was to be expected. By now, after monkeying with feed systems for over 150 years, designers have certain conceptual designs and design language they like to return to. For individual weapons, a box magazine is now standard. Except for some bolt-action sporting and sniper rifles, it’s a detachable box. For rifles, a double-row, 2-position-feed is common. (This was pioneered by the Thompson submachine gun), and the globally customary round count in a standard magazine is 30. For small pistols, a single-column magazine is still used, but service pistols generally have a double-row, single-position-feed magazine. (These pistols are, of course, apart from the holdout revolvers, which survive because it is a good, practical design for a defensive or hunting handgun, and out of nostalgia). The rough standard capacity of single and double-feed pistol magazines is 8 to 10 and 15 to 17 respectively, although there are outliers.

General purpose machine guns are generally fed from a link belt, usually a disintegrating link belt. Aerial and AA machine guns may use more specialized feed systems to deal with their higher rates of fire. Some of these systems use belts and some are beltless.

Military weapons may include provisions for loading magazines from stripper clips. For much of the 20th Century, stripper clips were used to load bolt action weapons, but semi-auto and auto-weapons that let you load the mag with strippers in situ were a transitional thing (like the FN 49 and the FAL). One US initiative borrowed from the German FG42 the concept of a stripper-clip guide to allow stripper clips to feed a magazine. We’ve now seen this in Russian service for 5.45mm ammunition — they probably think they copied the idea from us, and don’t know we copied it from the Germans.

By about 1920, some pretty standard ideas had developed. For instance, it’s generally a good idea to place the magazine as close to the mouth of the chamber as is practically possible, make the feed ramp as smooth, shallow and obstruction-free as possible, make the cartridge path as straight as possible, and make the feed lips (normally part of the magazine) as strong as possible. (Some of these objectives are obviously in tension with one another; you can only have a shallow feed ramp by having space between chamber and mag, for instance).

Unsupported ChamberYou also can overdo giving primacy to the feed system. This is likely a contributing reason that large-caliber Glocks are more prone to kB! than the original 9mm versions; the round path is so straight and the feed ramp so deeply cut that case support is borderline. You can see bulged cases from G22s and G23s (inter alia). Glock has changed the shape of their feed ramp to increase support in this area sometime since the early 2000s, and new Glocks no longer bulge cases. If you have an older Glock in .40 or .45, an aftermarket barrel like the Lone Wolf one shown in the picture (cribbed from Glock Pro) can prevent this bulge. For the record, Glock insists that the kB!s are due to overpressure rounds.

Some Unconventional Wisdom

Some weapons deviate from this conventional wisdom in interesting ways. For example, in the Tokarev TT-30 and TT-33 semiauto pistols and their derivatives, the magazine lacks feed ramps, which are machined into the solid steel of the removable trigger module. (Tokarev is one of the designers who really creatively improved on Browning, even if his trigger module was a dead end). And the Thompson Submachine Gun feed design is quite unlike any other magazine-fed weapon, and deserves extensive analysis. To begin with, the magazine is quite distant from the chamber; the magazine cut is 1.865″ from the front of the receiver, and the barrel only threads .500 into the receiver, meaning there’s a 1.365″ (plus the width of the mag itself, the mag’s setback in the magazine cut, and the round’s setback in the mag) distance from the mag cut to the back of the chamber.  This was a deliberate design decision.

David Findlay explains in his book Firearm Anatomy: Book 1: The Thompson M1A1 Submachine Gun some of the details of the Thompson design.

The Thompson compensates for its magazine position by having a long funnel feature that guides the round up from the magazine to the chamber of the barrel. This feed-ramp feature requires some unique machining and subsequent finishing.

tsmg sketches 101

To generate the feed ramp, a cutting tool must enter the front of the receiver at a compound angle and must rotate and sweep the feed-ramp cut at the same compound angle to generate the ramp in the picture below, the bolt is already stripped the cartridge on the magazine in the round still has not entered the chamber.

tsmg sketches 2

For most other weapon designs, the round from the magazine in the round position or move close to the chamber to aid feeding.

The Thompson’s design of the feed ramp, while very good from a feeding standpoint, is much more difficult to machine and, as a consequence, is more costly. In addition, having the chamber so far forward in the receiver makes clearing a “fail to extract” malfunctioning round difficult and makes the gun harder to clean. Feet-system design, though, is one of the most important aspects of any weapons performance. A great deal of testing must be done to ensure a good performance. Small variations and subtleties in magazine dimensions can have a enormous impact on gun reliability and function. The Thompson design teams approach compensated for different magazine configurations, and this type of design has stood the test of time.

Improving Feed Systems

Of course, the feed system a designer sketched in during the conceptual design of the firearm need not stay that way forever. Almost any firearm’s feed system can be improved. It’s one of the first places to look for problems if you’re experiencing jams, but it’s not just for malfunctioning pistols. Consider the millions that have been made by tens of thousands of gunsmiths, reshaping and polishing pistol feed ramps. A pro polish job can improve even a fundamentally reliable pistol like an issue 1911 or M35 Browning High Power.

In the 1990s we saw striking improvements in M16/M4 reliability because of three small improvements:

  1. The “M4 cuts” making a deeper feed-ramp and eliminating one cause of stovepipes and jams;
  2. Becoming ruthless about unreliable magazines, and discarding them as they’re supposed to be discarded when they begin to malfunction (there are things you can do to repair an M16 mag, but the juice is usually not worth the squeeze). This became a battle royal with the quartermasters, which was resolved in favor of the firearms operators when we began shooting up the bad mags so that they couldn’t be reissued.
  3. Better magazines, the first of which were the steel HK magazines that spread like a welcome virus (in units that did not have the discretionary funds to buy them, guys bought their own) and then in the Oughties the polymers led by Magpul.

Those things that you can really boil down to two changes, improved feed ramp and better magazines, took what had been a pretty reliable system and moved it up into AK “runs like tractor” territory, while retaining all the AR advantages in accuracy and ergonomics.


3D Printed & Homemade Guns Roundup

We haven’t done one of these in about 10 days, and a lot’s been happening, to include the State Department’s 1st-Amendment-negating attack on firearms data.

“You can’t stop the signal, Mal.” (Thanks for the reminder in comments).

The Political Rorschach Test

What do you see here?


Hmmm. I see. Interesting.

There are no wrong answers to a Rorschach. Just insights. Like… “You can’t stop the signal, Sen. Schumer.”

“Now, let’s go back to your childhood…”

Contemplate This, Andrew Cuomo:

NY Legal 3DPrinted Alimanu-Phobos

Does it offend you? It’s a New York Assault-Weapon-Ban legal and even SAFE1 Act Legal AR-15 based on the Fosscad Alimanu-Phobos lower receiver.

alimanu-phobosThe Alimanu-Phobos, designed by ArmaDelite, is based on previous lowers by JT, WarFairy and Shadowfall. (The Phobos, VanguardJT, and Vanguard lowers respectively). AD incorporated an ambi selector cutout at the suggestion of Crysis.

The .stl files are available on GitHub, at least until the Department of State gets its way.

The receiver is 3D printed, the other parts are NY legal and of traditional manufacture.

Because it can be built in a NY-legal version, it could be built in a CA-legal version with the addition of a bullet button.

Rep Steve Israel (D-NY) has expressed interest in a Chinese UN proposal to require government licenses for 3D Printers, and for printing anything on them.

Bolt-Together Receiver

The parts can be printed, machined, or both: 8 parts and 12 bolts = 1 AR-15 receiver, designed by RollaTroll.

RollaTroll Bolt Lower

buffer tower rear trunnionThere’s only one really complex part, the buffer tower or rear trunnion, seen here on the right:

This set of files, too, will make 3D printing an AR lower available to a wider and wider range of people and printers. You no longer need a fairly large format printer to execute a lower print.

There are obvious cons to doing it this way but the pros include: ability to farm jobs out, ability to use small printers (or use large printers for small, fast, interstitial builds), ability to combine materials and methods.

Here’s a version with the side plates rendered clear:

RollaTroll Bolt Lower clear

The adverse party does not get this: keep pushing bans, and you keep pushing innovation.

“You can’t stop the signal, Mr President.”

Then, there’s the accessories

TSMG AR GripLike a Thompson Submachine Gun Grip for your printed AR.

OK, that’s just whimsical. But you know what? The State Department would ban it. camera mount

And there are various customizable rails, suitable for duty as scope and camera mounts. Harmless? Not to the alleged diplomats.

“You can’t stop the signal, Foggy Bottom.”

Cody Wilson on the Latest Attacks on Defense Distributed

There are a couple of aspects to the Administration’s latest attack on Defense Distributed and the Ghost Gunner project. Wilson begins with a view from inside the Operation Choke Point stranglehold:

A mere two days after our April shipping announcement, DD’s longtime payments processor PayPal abruptly began refusing to process our orders and froze up many hundreds of thousands of dollars in our account while they invented a “terms of use review” for our product.

Many Ghost Gunner customers were personally emailed in the week thereafter and actually asked by PayPal for what purpose they intended to use their machine. I found both of these actions outrageous and condemned PayPal with the strongest language when I spoke with their risk department at the end of April, ending our relationship.

Now, PayPal is known to not be friendly to gun-related companies, this is true. And like our payments processor before them they acted like their hands were tied, but rather than cancel our accounts, they froze DD’s funds and then demanded a ten million dollar reserve before they’d process any more payments. This happens to be the exact sum for which the company was recently fined for their deceptive trade practices. Coincidence? I’ll let you decide. For my part, I told them to go to hell and refunded many of your orders.

Our break with PayPal also limited the usefulness of our order lookup system.

There’s more than that, but that’s the gist. It is, however, not the only way in which the politicians and the Deep State are attacking:

ITAR and First/Second Amendments

The news you’ve recently heard about the Obama Administration regulating speech related to guns on the Internet is true and actually worse than the NRA-ILA message has indicated. The State Department is now formalizing the power to police all public exchange of data related to any category of munition online. I’m frustrated with the NRA’s rhetoric, which is that ITAR has not yet been used to block websites or harm public speech. This is a lie springing from the organization’s refusal to develop a position on homemade and 3D firearms.

The State Department has in fact harassed and censored this company for two years with the ITAR. Every compliance move we make in an attempt to publish CAD files or gun software is re-routed, stymied or ignored. This Federal power grab isn’t a hypothetical, future scenario. It is already US policy and is being used to target politically-motivated speech related to the Second Amendment.

State has even directly threatened this company over opening our Ghost Gunner forum. The forum is literally closed to registration as of today because the US government doesn’t want us to be able to support an open-source DIY product expanding your 2A franchise. We are unable to publicly develop our machine with you until this agency has been corrected.

Because of this malicious set of laws, Defense Distributed filed suit against the State Department in early May. We’ve brought a First Amendment lawsuit and we are joined by the Second Amendment foundation and other legal talents in Texas. I want to emphasize that none of the gun lobbies are funding this effort and neither are they willing. Defense Distributed alone has has funded this multi-year endeavor and only by the success of Ghost Gunner and your patience were we able to afford to reach this filing.

I’ll update you all at the right time about this matter in more depth, but our current filings may be found here. Our first hearing is ordered for July 6.

I at least hope that with the NRA’s spotlight and the public comment period offered for the new regulations, the reg will be pulled. Comments will be accepted on the regulations until August 3, 2015, and they may be submitted online at or via e-mail at with the subject line, ‘‘ITAR Amendment—Revisions to Definitions; Data Transmission and Storage.”

But don’t be numb to just what this is. This isn’t about firearm industry compliance. It’s about divesting you of any free and public access to the means of your own defense; about pruning you from gunsmithing culture and its scientific exchange. Their claim is that gun technology is exclusively commercial and military. You’re a pleb and it isn’t for you.

Remember that your Revolution was fought for less.

Note that FOSSCAD IRC noted that:

Shutdown @DefDist’s CNC mill & users can get this: Eventually you wlll be banning electricity!

This image from Weaponeer Forum illustrates the issue.

Pseudo Colt 1911

The dark 1911 frame is factory produced, the light one is machined from billet aluminum — not with fancy CNC, but a good old manual machine. It has a ways to go but it will be an effective handgun.

This is why the Nork-enviers and Ayatollah rumpswabs in the State Department want to ban the information. 

Hasn’t anyone ever told Secretary Kerry: “You can’t stop the signal, John?”


  1. SAFE Act stands for Stalinism Always Fails Everywhere, if we recall correctly.


The Megapack is currently hosted on GitHub. To get all the files, the button you seek is “Download ZIP.” The files are licensed open source under the GNU GPL v3.

We don’t think the RollaTroll lower is in there. Patience. And confusion to our enemies, foreign and domestic.


There are several places to get the knowledge you need:

  1. The FOSSCAD IRC channel
  2. FOSSCAD’s twitter feed
  3. The Weaponeer Forum

Just to name a few. Let your imagination be your guide. Places to learn what’s going on, besides WeaponsMan of course, include

Those Who Forget the Past, AR-15 Edition

A couple of days ago we followed a link from The Gun Feed to the Michigan-based gun blog 248 (We’re guessing 248 is an MI area code? The way the Workshop Eating Plane® will have “603” in its N Number?). Anyway, the article was a short and to the point gear review of an extended or enhanced mag release that is made by a company called ArmaSpec.

ArmaSpec mag release

Armaspec calls it the “Tactical Combat Button,” and says (right there on the package!) that it gives you “faster magazine changes.” It’s reminiscent of popular extended or enhanced mag catches that have become common on sidearms, like the Vickers Tactical catch we have in our Glock 17. (We’ve got the slide release, too. Larry is a hero to those of us with small hands).

Apart from the name, which gnaws at us in its jejune buzzwordiness (not “tactical” again! And “combat?” Whose?), it illustrates the problem of living in Baby Duck World, where All Things Are Ever New™. This button may be useful for someone running very stylized match stages, but it probably isn’t.

Here are our problems with the TCB, conceptually:

  1. First, there’s nothing wrong with the standard mag catch;
  2. Unlike the standard mag catch, this is very prone to unintended mag release;
  3. Unlike the standard mag catch, this cannot be installed, removed or adjusted without tools;
  4. In fact, it needs a peculiar tool which the rubber-meets-the-road system operator may not have on his person;
  5. It gives up most if not all the adjustability of the standard catch;
  6. It introduces additional points of failure into a proven subsystem;
  7. It is vulnerable to the screws backing out and requires Loctite to work at all.

Note that we haven’t tried this part ourselves, we’re just cueing off 248Shooters’ review.

The History of the AR-15 / M16 Mag Catch

The M16 magazine catch started as the AR-15 one, which, of course, began as the AR-10 magazine catch, as shown here. (The first shot is Serial #38, auctioned by James D. Julia some time past; the others are from an Portuguese AR-10 on an H&H Semi receiver). We have not tested the interchangeability of these catches, but we suspect that the AR-10 and -15 catches are the same length on the X Axis (front to rear) but the AR-10 catch is longer on the Y Axis (left to right).

Julia AR-10 #38 serial ar-10_porto_right_side_receiver_rotated H&H AR-10 02

The AR-10 magazine catch was not created in a vacuum. It itself was an improvement of the catch used in the seminal German MP44 assault rifle (We use “MP44″ somewhat expansively here; the magazine catch appears to us to be the same in all related versions of the German assault rifle, back to the MKb 42 (H)). The direction of the release changed, and it was moved closer to the pistol grip, so that it could be released with the index finger of the right hand, instead of using the left hand as was done with the MP44. The next photos are of a Japanese non-firing replica of the MP44 (they were the clearest photos handy on the net). The mag release is the conical, ribbed button at the rear of the mgazine well.


This next picture shows a weakness of the MP44 system, which the AR system improved materially. As you can see, the catch, button and shaft are joined semipermanently by staking or riveting. That means it’s not field-repairable, let alone, -adjustable, at the -10 or -20 (operator or organizational repair, i.e. unit armorer) echelons. Again, this is a replica, but a very nice one.


By making the AR-10 design a one-piece shaft and catch, where the shaft threaded into a tapped blind hole in the mag-release button, Stoner made it possible for the magazine catch to be disassembled for repair, replacement or adjustment without tools. All you need is a cartridge to overextend the mag release so that the catch clears the magwell, and then it can be screwed in or out. On any AR, a mag catch that’s too “grippy” can be fixed by backing off a couple half turns, and one that’s kind of loose can be tightened up the same way. This adjustment can clear up a lot of “mystery” failures to feed in AR systems.

The magazine catch can’t unscrew itself without being overextended until it’s clear of the magwell, because the magwell holds the catch in place and prevents it from rotating. But as ingenious as the AR-10 magazine catch was, there were still two improvements to come.

The first was to exchange the blind hole of the AR-10 magazine release for a through hole. This made the magazine catch button much easier to manufacture and increased the usable range of adjustment for the magazine catch, with no downsides at all. From this alteration somewhere around 1960, the parts of the standard AR magazine catch are fundamentally unaltered until today. (One change is that the ribbing on the catch is circular, whilst in the early sixties it was straight and horizontal, but this is a cosmetic change driven by production convenience and not material to the function of the catch.

The 17 prototypes made all had a magazine catch that worked much like it has on all the milios of aRs since then. Here is Prototype 004, from the Reed Knight collection:

AR-15 Proto000004

The initial catch was not guarded at all.

Here it is on the Colt Model 601, the first production AR-15 model, of which approximately 14,500 were manufactured, mostly for military testing (project AGILE, SF/SEAL evaluation in Vietnam, etc.). This catch is identical to those seen on surviving prototypes.


One of the complaints from these early tests was that the exposed magazine release would occasionally lead to an uncommanded ejection of the mag while moving in thick brush.

The Model 602 (which is labeled “Model 02″ on the left magwell) was purchased in about 19,000 units, primarily for Air Force base defense and plane/weapons guard purposes. It has the same arrangement of slabside receiver and mag release button. It was with full rate production of the M16 (USAF rifle,  Colt model 604) and XM16E1 (US Army rifle, Colt model 603) that another change to the receiver made it possible to guard or “fence” the magazine release.

The change was the substitution of a captive pivot pin, retained in the lower receiver by a spring-loaded detent running in a groove, much like the rear pin, called the “takedown pin,” had always been. A boss needed to be added to the lower receiver, to provide a race for this pin’s detent and spring to run in. Since the forging dies needed to be modified anyway, it was relatively trivial to extend the boss and make it a “fence” riding above the magazine release.  (This is the center receiver in the three-image picture below). Now, bumping into a stand of bamboo didn’t mean a lost mag any more.

Except, reports from the field indicated that it still did. As a result, the users — mostly the Army, based on Vietnam experience — asked that the rifle be modified, again. The request was brought to the Rifle Technical Committee on 13 Jan 66. It was feasible to change Drawing No. 62300 for the M16 and XM16E1 common lower receiver forging, as the running change log of Product Improvement Modifications records, “To respond to Army request to provide protective boss around the area of the magazine.” The Army contracting office approved the change on 16 May 66, and sometime relatively soon after that date the forging dies were modified to incorporate the “protective boss” which has since come to be known in the collector community as the “full fence.” A comparison of the three different receivers, showing the different forged outer right magwell side, is below, based on thumbnails at the Retro Black Rifle site (which also provided some of the other photos, although the AR-10 photos are from Julia Auctions and from the collection).

Left: prototype through Model 602. Center: Pre-March-66 603/604 (XM16E1/M16). Right: post-3/66 603/604 (XM16E1, from 68 M16A1/M16)

Left: prototype through Model 602. Center: Pre-March-66 603/604 (XM16E1/M16). Right: post-3/66 603/604 (XM16E1, from 68 M16A1/M16)

All the earlier forgings were used by Colt; those that were machined already seem to have been used until they ran out on military 603/604s, some were retained for toolroom prototypes and other factory uses, and slabsided, early model forgings with different machining (for a pivot screw instead of a pin) continued to be used on civilian-market semi-auto SP1 rifles for over 20 years.

The fenced mag release solved the problem. It is very rare (a freak occurrence, in fact) to have some stick or branch (or interaction with other gear or aircraft structure, etc.), drop your mag. And yet, there’s no difficulty reaching the mag release with your right index finger and dropping the mag free for a rapid reload. (At least, if you’re right-handed. Yeah, the ergonomics are significantly worse for a southpaw).

Why All this Ancient History Matters w/r/t this Rifle Accessory

The saga of the growing “fence” or boss on the receiver’s magazine well is the story of successive responses to a real problem, inadvertent and uncommanded actuation of the magazine release. You might say the military found that a protected switch was a “tactical” and “combat” necessary, and their users were actually, not Walter Mitty, tactical, and really, not in a practical-shooting-competition stage sense, in actual combat. And they decided a protruding magazine release was a A Bad Thing®. Enough, indeed, of A Bad Thing® that they spent the money not once, but twice, to redo the lower receiving forging to insulate the user against the consequences of a protruding button.

And here’s what the Tactical Combat (gag me!) Button looks like, installed, close-up (this nicely-done image is from 248Shooters’ review, we don’t know if they took it or it’s a factory shot):

ArmaSpec MR close-up

As they do note, it’s a well-made small unit, but by installing it, we not only have resurrected the inadvertent mag-drop failure mode, the one that was supposed to be laid to rest in March of ’66, but we’ve also introduced a new failure mode, in that foreign matter can potentially get stuck between the large pad of the TCB and the side of the receiver. In fact, the receiver boss/fence could actually help entrap a vine, stick or other junk right where it keeps you from pressing the mag release down.

This is apart from two of the cons noted by the 248Shooters reviewer, that the screws need to be Loctited, and that, “Like most extended mag releases it does fall pray [sic] to having a bit of wobble.” Against that, we dragged M16 series rifles through Arctic and Alpine conditions in places like Canada, Norway, all over northern New England, and some of the 20k peaks of the Andes, and the factory release is readily manipulated with gloves and even with mittens.

One reason we harp on this design history is that you have to know why the designers designed features into the platform before you go redesigning them, lest you bring back failure modes that engineers thought they banished fifty years ago.

Just like when you hot-rod a car, you may change characteristics that were designed into it for a reason, you need to think before you hot-rod a rifle. If you’ve ever had to drive an undercooled, over-cammed, 12:1 compression race car in traffic, with a did-you-do-your-squats-today clutch and square-cut gears, you know what we’re talking about.


This sort of post is the kind of technical information we most like providing. But the US Department of State has moved to require prior restraint — Censorship, with extremely expensive licensing subject to arbitrary terms — on firearms technical information, in a wild grab to stretch the International Trafficking in Arms Regulations far enough to snuff out freedom of speech. (We’ll have more to say about that soon, including suggestions for how you can help, but from now on until this monstrous and deviant interpretation of the law is put down like a rabid coyote, every technical post will incorporate a note on this subject).

Repairing a Broken Firing Pin

Ever broken a firing pin? If you’re like us, you have, and then you either ordered a firing pin replacement, or had a gunsmith make one, if there was no factory or used firing pin to be had. Or maybe you just hung the weapon back in its place, your equivalent of the “too hard” file on our desk. This excellent video from the American Gunsmithing Institute (yeah, those guys that want to sell you approximately a million videos for approximately a million dollars) has gunsmith Ken Brooks in show-and-tell mode as he restores a firing pin — in this case, for a Winchester Model 1894 .30-30, but the principles apply whether your broken firing pin is in a Luger or a Lewis gun1.

It was rather eye-opening to us that he applies no heat-treating to the finished part. The spring-steel stock he uses is already heat-treated, of course.

The most valuable information in his entire presentation (which we watched raptly from end to end) was his description of the specs for firing pin fit and protrusion, and his reduction of these to simple rules which work not only for low-pressure blackpowder-era cartridges like the .30-30, but also for high-pressure modern rounds.

Next was probably his warning of the necessity of washing off soldering flux. Fine and good to solder the new pin in place, but his description of what happens if you let your customer go tripping out the door of your shop

You can extrapolate from here to some common firing-pin related problems you may have encountered. Light strikes or no strikes? check protrusion. You could have a short tip, or a firing pin held back by corrosion.

On some frequently-broken firing pins, Ken’s techniques won’t apply, for example on the simple turned firing pins in Tokarev rifle and pistol designs.  (Both firing pins have a design weakness, in that they have a large cutout for a retaining pin. Coupled with wartime manufacture that short-cut deburring and polishing, they are very prone to breakage at the corners of that recess. A little bit of bench work is in order if your firing pin has sharp edges or other stress risers in this area). But those pins are relatively easy to make, if you have a lathe.

Yet it turns out that this kind of pin is, if not easy, quite straightforward to repair. Repairing an original part detracts less from a gun’s originality than replacing a part, which may be a factor

Hat tip, the NRA.


  1. Hmmm, we might have overstated that. We’re not sure there’s enough material in a Luger firing pin (which is mostly hollow) to replace the nose and meet Ken’s at-least-half-inside requirement. And a Lewis gun’s firing pin is part of the operating rod, and if it does break, may break where a soldered-spring-stock repair is not practical. Fortunately, it’s overengineered and seldom breaks.

Update, surprisingly related:

The NRA reports that the State Department is trying to ban the internet publication of technical information like this and other material hosted at this blog. This is not surprising from those Game of Thrones wannabees. We intend to carefully read the proposal (.pdf; the State Department wall of verbiage begins in the lower right corner), run it by our ITAR consultant in the day job, and react accordingly.

We guess: the gun-banners in striped pants at Foggy Bottom are not going to like our response.

How Many SIG Pistols are in This Picture?

Care to guess?


The answer is one. What you’re looking at is the first two-size, two-caliber conversion kit we’ve seen for the SIG P320 striker-fired pistol. (The earlier P250 is a similar modular concept, but in a traditional SIG hammer-fired model). This kind of package has been announced by SIG, but this is the first one we’ve noticed for sale. The giveaway in the picture is the absence of a trigger in the black pistol.

That’s because the actual serial-numbered receiver, and therefore the firearm is the fire-control module. Interchangeable backstrap not enough for you? How about a whole interchangeable pistol?


In this particular case, the black pistol is a full size 9mm pistol and the .357 SIG is a compact. Numerous other combinations are possible with additional parts.

One benefit of this combination is that it makes it possible and practical to practice with relatively inexpensive 9mm ammunition, saving the expensive .357 warshots for limited confidence/function shooting and daily carry while maintaining a single array of muscle memory on grip, sight picture and trigger pull.

At $800 the set, it saves you some money over a pair of SIGs or even a pair of lower-priced Glocks, if you or our agency want the .357 SIG round as a carry round.

Personally, we’re content with the venerable 9 x 19, but enjoy the creativity and the technology of the thing. This would likely be a very hot seller, if SIG can overcome their reputational struggles with quality control.

3D-Printed 9MM Semiauto (video rich); 3D Guns Update

As we have expected to happen for some time, and as the initial Cody Wilson “Liberator” first demonstrated, 3D-printed firearms made of common addititive-manufacturing plastics like ABS or PLA inevitably had to diverge from common steel firearms practice to take advantage of those plastics’ strength — and overcome their weaknesses.

That means that, while early prints were nothing but, for example, a plastic version of an AR lower dimensionally identical to its aluminum forbear, but destined for a short life (especially in PLA), more and more designs are innovating in different directions.

This series of videos shows the Shuty, a 9mm pistol based on kitbashing the designs of British homemade gun pioneer P.A. Luty and the AR-15 together. It uses several metal parts, including the barrel (which comes from a Glock 17), the fire-control group (AR), and the bolt (home-made). On the other hand, the magazine, upper and lower receivers, and bolt carrier, are all printed from a polymer generally thought unsuitable for firearms parts. Turns out, you can design around materials deficiencies (as the Japanese did when they used chrome bores for strength, to offset the suboptimal alloys they had for rifle barrels, decades before other nations adopted them for durability, and when their aeronautical engineers designed assemblies built-up of  7075-equivalent alloy sheet where every other skyfaring nation would use a 7075 forging).

Here is Derwood’s working Shuty, redesigned from the original, as of 1 May 15:

He says:

After several failed attempts with the Shuty, I decided to beef it up to handle the stress. The combination plastic/steel bolt works very good. After several test fires, the frame and lower is holding up well and no damage has occurred.

The plastic parts were all printed on the SeeMeCNC Orion printer, an entry-level machine, in PLA (polylactic acid), the entry-level printing material that is biodegradable and derived from renewable resources. The bolt assembly looks complex, but:

Its just three steel dowels stacked and welded together parallel with each other. the bottom smaller dowel is drilled for the firing pin. the center dowel is a spacer. the top dowel is the buffer.

Fosscad (an informal, leaderless, cellular homemade-3D-gun resistance) picked up the video and Fosscad user ma deuce posted it on 22 May 15. (Link only because it’s basically the same video, why embed it?)

Here’s Derwood’s next video, 20 May 15, showing a longer test fire. What appears to be a jam at the end isn’t, actually; what it is, is the bolt gnawing on the magazine spring because this work in progress doesn’t have a magazine follower yet — just a spring pushing the cartridges up! Oy.

Well, if you’re going to crib something, cribbing Glock’s feed ramp by using their barrel is a short cut to a working firearm. Glock reliability is not accidental, it’s a product of careful design and iterative improvement.

So that brings us to 27 May 15. It’s fully working, with firing and a mag change, two eight-round mags complete:

Derwood says it’s still evolving, and not finished yet; when he thinks it’s “finished,” he’ll release the .stl files. Until then, he tinkers on at a high rate of speed.

As a practical 9mm pistol the Shuty has its limitations. It gives you all the firepower of a Kel-Tec belly gun in a platform the size of what it is, a mongrel of AR-15 and MAC M10 ancestry. It has no sights, no stocks, and is only slightly more concealable than a basketball. Made of PLA, the stuff used in the dishes microwave dinners come in, it’s destined for a short life, by gun standards (we’ve got guns one and two centuries old here). So, as a practical pistol? A turkey. But as a proof of concept, it is enough to get would-be totalitarians “all wee-wee’d up” (in the locution of one such).

Ah, but bolts? Barrels? Too early to write about, but people are working those issues.

Some Other 3D Developments

Of course, the Shuty is far from the only 3DP pistol in development. Here one is with the Imura revolver (left) and the Songbird pistol (center):

Imura Songbird Shuty Redesign

Joel Leathers of Texas even posted the .stl files for the Glock 17 on Thingiverse. (That link 404s; the files were deleted, due to MakerBot’s political anti-gun position, but there is a story on At least they didn’t unperson Joel on Thingiverse. Yet).


Of course, a printed Glock part will not be usable in a firearm as is. But we can see practical uses for the files. (How about a printed, brightly colored, safety barrel for use in mechanical training? Pennyslvania State Police?)

How has this technology progressed so fast? Some of these guys print a lot. This printer has racked up nearly two months of run time, and used over six miles of filament!

Some of these guys print a lot

This is a 10-22 with receiver and trigger housing printed. We’ve discussed this project before. (Indeed, that story from last month has a photo in it which is a crop of the one below).

10-22 with major parts printed

We’ve shown the receivers before, but here are some printed trigger housings.

10-22 trigger housing printed

AR receivers continue to be developed. This heavily-reinforced AR-10 lower design, the Nephilim (an obscure Biblical reference to a purported race of human/angel crossbred giants) by Warfairy, shows lots of reinforcement and improvement to make a plastic receiver stand in for a 7075 aluminum alloy forging.

AR10 Nephilim by Warfairy

Here at Hog Manor, we’re still on the waitlist for our printer. And we won’t be printing guns with it, but other stuff for our DOD clients.

If You Build It, Nanny Wants to Ban It

Banning this sort of thing is very tempting to anti-gun lawmakers, political appointees, and those executives in the ATF who see the agency’s mission as “to destoy gun ownership.” Indeed, some of the European nations with fewer checks and balances hindering their legislative range of motion have already banned this kind of experimentation.

The problem with that, is that it is but a short step from the Shuty to a select-fire submachine gun. If you drive this design activity entirely underground, the designers are as well hung for a sheep as a lamb, no?

The largely-libertarian tinkerers making these things are doing no harm to a society, and may do some good. They have no sympathy with criminals who would use this technology to harm or threaten people. But let that be the line the law draws in the sand: not the malum prohibitum “if you make this we will hammer you,” but the malum in se “if you do harm with this we will hammer you, and the maker community will help us find you.”

VPO-208: Russian Gunsmiths Respond to Russian Law

We’re familiar, here in the USA, with weapons that are shaped by US gun laws. We have a variety of weird and wonderful arms that exist only because of the Gun Control Act of 1968, the National Firearms Act of 1934, and the patchwork of implementing regulations and executive orders that have shaped the US market. In addition, state assault-weapon band have resulted in oddities like California’s “Bullet Buttons.” A wide range of legislatively-midwifed Frankenguns, from the Walther PPK/S, to short barreled rifles, to pistols with SIG braces, reflect the degree to which designers are constrained by the gun-designing impulses of American politicians and bureaucrats.

It should come as no surprise that the same thing happens in other countries with large gun markets. This case in point comes to us from Russia, where gun laws are generally stricter than in the United States. There, no one can own a pistol. Most citizens can own a shotgun; but to own a rifle you have to have owned the shotgun without incident for five years.

So here comes the VPO-208: an SKS shotgun.

SKS in .366Produced by Techcrim, an Izhevsk manufacturer, the .366 by Russian measure, across the lands (.375 by ours, across the grooves), is a smoothbore or near-smoothbore gun that gets the would-be gun owner into a semi-automatic, service rifle platform, while staying within the letters of Russian law.

The ammunition appears to be made from fireformed 7.62 x 39mm casings, and is available in a range of sporting projectiles, plus a shotshell variant.

It is reminiscent of such American wildcats (some of them since turned production) as the small-head .300 Whisper, .300 AAC Blackout, .338 Spectre, and the Mauser-head-sized .375 Reaper, all of which run in the AR-15 platform. It just goes to show that this kind of innovation is hardly an American monopoly.

The first table in the advert below has three columns: “Type of projectile”; “Speed, meters per second;” and “Energy, Joules”. Here’s our conversion of this table.

Projectile Type Velocity, m/s Energy, J Velocity, fps Energy, ft-lb
LSWC poly coat 13.5 grams 640 2765 2099 2039
FMJ 11 grams 700 2618 2296 1931
FMJ 15 grams 620 2883 2034 2126
JSP 15 grams 620 2883 2034 2126



As the shot of the fired JSP shows, and these velocity and energy tables suggest, it would actually be a good short-range hunting round.

The second table, with the bullet-drop diagram, is, “Velocity and Energy of Projectile, .366 TKM with 15-gram FMJ bullet”. Here’s our translation and unit conversion.

Metric (SI) Values Muzzle 50 meters 100 meters
Bullet Drop mm 0 35 125
Velocity m/s 625 570 520
Energy J 2837 2437 2028
English Values Muzzle 50m 100m
Bullet Drop in. 0 1.38 4.92
Velocity f/s 2050 1870 1706
Energy ft/lb. 2092 1797 1495

The problem with the gun is its accuracy, as it’s basically a smoothbore. Hyperprapor suggests that it might be minute-of-E-silhouette at 100m.

But hey, it will let some Russian guys own the rifle their nation’s color guards parade with, and even let them shoot it, all with the reduced paperwork and hassle of a shotgun; perhaps a big win for them.

There are no ballistics for the shotshell, which exists, we suspect, primarily to navigate the channels of Russian weapons law. (This law does seem somewhat liberalized since Soviet days). Techcrim’s website shows that they are very active in small-caliber (.410) shotguns and shells, which seem to have more of a following in Russia than they do here. We wonder if that’s an artifact of Russian law, too.

We saw this on r/guns, posted by our old friend hyperprapor, who notes that under Russian law “paradox rifling”  is legal if it’s under 150mm long (About 5.9″).  Paradox rifling is rifling that was just engraved in the last few inches of the bore of what was otherwise a shotgun, to give it some capability with a single ball or bullet. It was named by English bespoke gunmaker Holland and Holland, who adopted the patent from GV Fosbery of Webley-Fosbery fame. Westley Richards called it “Explora” but other makers seem to have stuck with the paradox name.

And this is definitely one for the “how weird does it get” file — a smoothbore SKS that is one short hop removed from the Webley-Fosbery Automatic Revolver!

You can’t drill a #40 hole with a #40.7 or so Drill

OK, not gunsmithing here, but planesmithing. And something has been going wrong for a while, but we were too blind to see it.

bent drill bit

The first phase of building a RV-12 involves drilling a lot of holes. It’s not arduous; many of the holes are already drilled in the parts, although undersize; hundreds need to be finish-drilled to size; some parts need to be clecoed together and match-drilled so there’s zero relative movement when they’re riveted together; a few holes need to made in virgin metal. Once the holes are drilled, they need to be deburred (we use a single-flute deburring tool) and some of them need to be countersunk for flush rivets. (Some, in thin sheet metal, are dimpled instead of countersunk).

In the tail section, relatively few drills are used. Because the AN rivets and pulled rivets are expressed in fractional sizes, most of the holes are drilled with numbered drills, and the ones that got a workout were #12, #30 and #40. (Drill gauge is like shotgun gauge: larger numbers are smaller diameters). For instance, the right hole for a 3/32″ rivet (0.09375″) is not a 3/32 (no clearance), but a #40 (0.098) gives you about four thousandths for clearance — and tolerances.

We found making holes to be easy, but there were a lot of frustrations when it came to using those holes. We couldn’t get the countersink’s pilot in the hole all the time (we wound up using an unpiloted countersink). Clecos, the small spring-loaded pins used to temporarily join sheet metal destined to be riveted, didn’t always go. Some of this is tolerance in the Clecos, and we through some troublesome ones away. (You can never have too many Clecos when building an aluminum airplane, but bad tools need banishing or they just cause trouble). Then, when it came to riviting… the rivets didn’t fit. It was hard to fit them in. When we did, the friction was so heavy it was hard to form good heads.

On the bright side, we got some practice at drilling out rivets. So there was that.

Last night, we had an epiphany. So many of our holes were undersized, yet, you could slip the drill bit right through no problem, so we hadn’t made the holes with the wrong bits or anything like that. But we had always been using our new Hertel drill bits, the same ones we made the holes with, as measuring tool. We took up an older #30 and tried to slip it into the hole the new #30 had made, because the AN rivet spec’d for the hole wouldn’t go.

Neither would the older #30 bit. Oh, crap, were our new, high-quality drill bits undersized?

We didn’t want to think so. Therefore, we measured all three of the ones we’d used a lot.

Drill Bit Nominal size Measured size ∆ size
# 12 0.1890 0.1890 0.0000
# 30 0.1289 0.1170 0.0119
# 40 0.0980 0.0940 0.0040

(Note, we’re not claiming that our digital caliper technique is accurate to four decimal points. We just added zeroes where needed to make everything line up).

The bad news: we’re going to have to redrill every freaking #30 hole, and some of the #40s as well. (If a rivet test-fits without pressure, we know we’re good). The drill bits were actually undersized, #40 by four thousandths, and more than one hundredth.

They didn’t get that way from us using them — we’re not cutting anything but sheet 2024 aluminum alloy.

The good news: we now know why we were bedeviled by non-fitting rivets.

The moral of the story: from now on, test drill and check for size using a gage pin, every time we change sets of drill bits.

The company that made the drills is Hertel, and it was a quality (and pricy) set. The supplier was MSC. It will be interesting to see what happens when we call them with this problem.