Snap, Crackle, and Pop

Well-known (and respected) trainer Kyle Defoor was conducting training at for a military unit when one of the unit’s long guns went down, due to this:

defoor bolt failure

Yes, that’s an AR/M16/M4 bolt with a single lug fully failed. Possible causes for the failure include (at a fundamental level) manufacturing error, corrosion or fatigue. It’s hard to judge from this hole, but going way out on a limb, it looks like there’s a somewhat granular failure at the left end of the fracture, with a smoother “sudden” fracture face on the right end nearer the extractor, presumably because the fatigue failure left too little of the remaining metal to bear the stress of firing locked in battery, and the remainder of the part failed from the crack due to overstress. But it could also be caused by swapping a fresh bolt into a gun with a worn barrel extension (or vice versa) in the field, so that only one lug was bearing all the tension of locking — result, failure. Or the gun may simply have been made without the locking lugs all engaging properly — it’s happened before.

A gun with a failure like this may or may not continue to fire for a while. But if overstress on one lug was a factor, the loads formerly too much for seven lugs now bear upon six — it would not be wise to bet your life on this firearm.

Kyle, though, had another issue with the failure — and the unit whose arms room coughed up the firearm that did it.

On 9 July, he posted this image to his Facebook feed, saying:

Maybe I should start to amend contracts to include an armorer and spare parts?

With a hilarious set of hastags including, but not limited to:

#‎takecareofgear‬ ‪#‎ittakescareofyou‬ ‪

…and the snark-infused:

‬ ‪#‎logisticswinswars‬ ‪#‎waistingtrainingtime‬ ‪#‎youdontpaymetoplumb‬

The part was, as you can see from the markings, a factory Colt, magnetic particle inspected, bolt (or a counterfeit thereof that somehow got into the supply system — not impossible). It had unknown hours and rounds, because Big Green is not in the habit of keeping meaningful usage and maintenance records on small arms.

Apart from spelling “wasting” wrong, there is not much to argue with in Defoor’s response. Apparently the unit in question did not provide an armorer for the range event. In most units, the armorer doubles as a supply clerk and is not thought of as necessary for a range evolution (except to manage draw and turn-in of weapons at the Arms Room). In addition, the Army has been working to reduce the number and kind of spare parts available at organizational level. This is due to politically anti-gun policies, and Army civilian political appointees who believe (however lacking the evidence may be) that Army stocks are a significant source of crime guns.

Even if the parts were by some miracle on hand, the standard Army armorer, one each, is neither trained nor authorized to replace a failed bolt. Armorers given scant and cursory training on maintenance.  Instead, their course, an add-on for supply clerks, concentrates very extensively on paperwork, records-keeping, and the process of appearing to be conducting scheduled maintenance. This is also borne out by what actual combat units and their commanders value, based on how they judge and critique their armorers. No one is ever graded on the only maintenance measure that ought to count, the combat serviceability of the unit’s firearms; everyone is constantly graded on the process, on the appearance of maintenance, and on maintenance busy work. While we’d bet nine out of ten of the readers of this blog could fix this rifle in minutes, the only thing a company, battalion or even brigade armorer can do with it is turn it in.

Military maintenance bureaucracy does all it can to limit effective maintenance of small-unit equipment, notably including small arms, optics, and radios. Problems with these are most effectively solved by trained, experienced personnel at the lowest organizational level, so naturally such personnel are just flat not available.

Instead, you must tag the weapon or other piece of equipment down. Naturally, there are different rules for weapons and weapons equipment, vehicles, radios, and special weapons (i.e. WMD-related stuff), although the Army does try to squeeze them all onto standard forms (DA-2404 for regular maintenance, DA-2407 for turn in, nowadays it’s an electronic form, DA-2407E, done in the SAMS logistics computer system).

The weapon can’t be sent directly to the level that can fix it, even when (like this) the level is obvious and the weapon could be inspected and classified by a well-coached Helen Keller. It must go up the operator-organizational-direct-depot support chain, getting a new inspection at each

Plus, while the weapon is turned in, what is Joe Snuffy supposed to shoot? No Army unit maintains operational floats or spares (unless it is, by happenstance, or the customary incompetence of all Army personnel managers and activities, understrength). So Joe will get the weapon of whoever is on sick call or leave when the unit goes to a range, unless it’s one of the very large number of units that does an absolutely crap job of tracking who is assigned each particular weapon, in which case it’s musical chairs and the last one that shows up gets a new weapon.

The Army actually tries to bill giving a guy a new rifle for every annual, semiannual or quarterly trip to the range as a plus, believe it or not: “Everybody gets valuable experience in zeroing.” (Meanwhile, of course, everyone loses confidence in the ability of his gun to hold zero).

It does not help that the standard M12 rack does not accept a rifle with optics. In the Arms Room, it’s still 1988.

Moreover, the Army’s weapons records are a chaotic mess of rack numbers, serial numbers, weapons cards, hand receipts, pencil sheets, green-and-white property book printouts (that may not put all your unit’s rifles, for example, together on the same pages), and unofficial Excel-spreadsheets and Access databases, which interface more or less (mostly, less) with one another and with the unit’s personnel assignments. This means that every time you cross-level personnel from 2nd platoon to 3rd platoon, if your arms room is nicely organized by platoons, Joe Rifleman is going to get a new rifle and be off zero until next range trip, and so is Bill Bulletician who’s coming from somewhere else… that’s another reason why no Army unit beyond the Ranger battalions and the 82nd Division Ready Battalion actually dares to ship out to combat without a trip to the zero range.

In addition to the deployment delays that come because no one has confidence in his optic zero right now, we also endure a colossal waste of time because weapons inventories are unnecessarily hard. (One of the nice things about HK 416s? Their serial numbers are highlighted. Seems like a small thing, until you’ve tried to inventory a couple hundred M16A2s by the light of a flickering fluorescent bulb that there’s no budget to replace. And if you highlight the number with paint or permanent marker, you can actually get dinged on inspection). Every arms room needs to be inventoried periodically by senior personnel who have better things to do, and many aperiodic inventories are demanded by regulations. The faster these go, the better for everyone, but the Army has a settled way of doing things that proceeds from the assumption that the net value of a soldier, NCO or officer’s time is always zero.

 

36 thoughts on “Snap, Crackle, and Pop

  1. DSM

    Wow, different culture and procedure. New bolt, headspace gage, pin protrusion gage. Back up and running in less than five minutes. Without knowing the round count on the barrel and any maintenance history I’d be inclined to replace just the bolt. If the weapon record showed a previous busted bolt then the barrel would be scrapped. All things a fresh out of school CATM troop can fix in the shop w/o supervision, and still log the appropriate forms. I’m glad the AF got that piece of the puzzle right because that must be maddening.

    1. Miles

      ‘Big Army’ will usually have no record of prior repair done to a weapon, unless the armorer happens to remember it.
      SAMS-E doesn’t track ‘history’ except for last cyclic service date unless the new system I was hearing about before I retired has that feature.

  2. John Smith

    I have had a similar failure in a Mk. 18. The rifle in question had a fairly significant round count with no previous similar failure. Interestingly, as you stated, the gun continued to do its work until I took it apart mid range day to see why the gun felt…..well… “lazy”.

    (Lazy meaning that I could almost feel the individual components of the cycle as opposed to the feeling of a single unified process. Hard to explain.)

    Anyway- the little donkey gun was retired to the armor’s capable hands and I was stuck with a spare until effective investigation and repair could be executed. Long story made short- the answer I was given was: “it broke, we fixed it.”

    I’ll be sure not to do that again….

    1. Hognose Post author

      Pretty sure that redundancy was one of the reasons given for Johnson’s original bolt design patent, but I’m too busy to look it up now. I’d love to see macro photography of the fracture face of both parts. That would tell us a lot about how it failed and why.

      That you knew your Mk 18 had a high round count put you miles ahead of the guys Kyle has been training. But they probably need the training really badly.

      1. John Smith

        Redundancy stands to reason- I have heard an increased surface area conversation but have never been able to rationalize it completely.
        Round count is largely a function of being assigned an individual weapon and then holding the shooter accountable (hearing this one more and more…) to keeping an accurate record. Draw weapons are like rental cars….and you know how your average 22 year old takes care of those.

  3. Docduracoat

    As a 20 year gun nut who has broken his fair share of AR 15 bolts in various calibers and always just replaced the bolt, I was educated by this site that merely swapping out bolts is not the proper procedure.
    The AR 15/M 4 headspace is set by the factory when the barrel extension is torqued onto the barrel. There is nothing a home builder or Armorer can do to adjust the headspace on a chrome lined barrel. On a new build, most people do not bother to check the headspace. For the o.c. among us, the bolt with its’ ejector and extractor removed should close on a go gauge on a new rifle and not close on a no go gauge.
    If you are a Marine, when the bolt breaks, as it will after a certain number of rounds, a different bolt is acquired. The bolt without the ejector and extractor, should close on the go gauge, but not close on the no go gauge. If it closes on the no go, the only option is to try other bolts. If none will work, then that barrel is scrapped as the headspace is too long.
    If you are in the Army, you use a slightly longer gauge called a field gauge. Same thing, stripped bolt should not close on the field gauge. If you can’t find a suitable bolt, that barrel is scrapped.

    1. Miles

      To clarify a point.

      The military only issues one headspace gage for the M16/4 series. And yes, it’s dimensions are within the range for a ‘field’ gage.

      If a Marine unit is using ‘GO’ and ‘NO-GO’ gages, they are using the gages that were issued for the M27 (HK 416) rifle. Not that that’s bad.

      However, in my experience, the use of a ‘GO’ gage in M16/4 & HK 416 rifles has never been useful after the service on receipt inspection.

  4. Big Country

    Hognose:
    Dead on sir vis-a-vis Mainenence or proper lack that thereof. the number of qualified slots for 91F Small Arms Repairmen for the State of Florida currently number 4. and as of the end of this week, because I can’t permanantly relocate to Jacksonville, the number of QUALIFIED Small Arms Repairmen will be down to 2 (the guy in another unmentioned location is a Wheeled Vehicle Mechanic who was hired as a S.A.R. b/c he’s assh*le buddies with the CSM. NOT because he’s a qualified 91f. He’s had the “quickie” two week unit armorers course which means he’s strictly capable of doing a Dash 10 PMCS and -maybe- cleaning a weapon… however having worked with him, I have to say he’s a total eight ball who doesn’t know one end of the weapon from another.

    Another thing to touch on: You mentioned the budget? Let me tell you… Big Green is penny wise, dollar stupid. They just fielded the M2A1 .50, the M320 40mm grenade launcher and the M-26 MASS and when I put in for travel orders and such, I was told it was out of the budget, and that I “didn’t necessarily need the training” O RLY? Then how in the hell am I supposed to certify and repair these if and when they come across my board?

    As you said, the only thing they care about is t6he APPEARENCE of functionality… even if it gets someone killed.

    1. Miles

      Four (4) ?

      Daaaayumm, that sucks.

      I will always be grateful for the units I was assigned to.

    2. Hognose Post author

      Big Country: Kyle comes out of the Tier 1 world so he’s never seen how it looks out in Big Green (or in his case, Big Haze Gray, if I have his bio right). I will have more on maintenance at 0600 tomorrow.

  5. aczarnowski

    All my 5.56 uppers were built by reputable companies but this got me adding a no-go gauge to my AR tool set just to see what I can see.

    What came after the dangling “Moreover,” at the end?

    1. Miles

      If you haven’t already, add a commercial ( not military ) throat erosion gage, and if you can find one, a barrel straightness gage to your tool set.

  6. Kirk

    Don’t get me started on the screwed up Army policies for running maintenance.

    Supply clerks are supposed to be the armorers, but there is no way that they can do the job, which should be a dedicated, full-time slot in any unit with more than a couple of M16s and a smattering of pistols. They should be trained in basic weapons maintenance, but, no, that’s too much trouble.

    The Army is, I’m afraid, not a truly “armed” service. It’s a bureaucracy with some guns, and so far as the average leader is concerned, the nasty black things in the Arms Room are there to serve as costume accessories for when they have to take the troops out to play soldier. In my 25-year career, I ran into precisely ZERO fucking officers of any rank that were even hobbyist gun people. Every single time I went out to the range we weirdos that wanted to shoot weapons were allocated for off-duty use, I ran into enlisted, former enlisted, and civilians. No officers, ever. Makes ya wonder, don’t it?

    I could fix the system, but that would mean that the people running it wanted it fixed. They don’t, so it won’t ever be. At a fundamental level, they don’t see small arms as being at all important, nor is proficiency in their employment seen as a big deal. I was sitting in my office one afternoon, running some stuff down for the S2, when I got the message from one of my guys that the M2HB range they were at was an impending disaster because the fucking SFC that they’d put in charge of it didn’t know enough about the M2 to run the range. Dude did not even know that the weapon required headspace and timing gauges, and hadn’t taken any out to the range. They were trying to do the old BS trick with the stacked coins to make it happen–On a stateside range. There were other issues, but my guys wanted to know if I could dig out the gauges we had on hand and run them out to the range. After I hit the ceiling, since they’d been out on the ranges since 0630, and it was then around 1400 and they still hadn’t gotten the first firing order through qualification, I went down the hallway to the Brigade CSM and dropped a bug in his ear. Detonation followed.

    Now, herein is an illustration of the problem: A SFC in a combat arms MOS did not know how to run an M2, let alone a qualification range for it. And, the CSM from the Brigade, right along with the CSM for the battalion running the range, were not out on the range checking on things. Until I got them spun up by going down and pitching a bitch in the Brigade CSM’s office, both of them had more important things to be doing that day than go out and check on a range.

    Further, there were no repercussions for the SFC running that range. None. He was just told to go review his manuals, and do a better job of preparing for a range. In my book, his career should have ended, not the least because of the unsafe manner in which he had things set up out there–There were limiting chains on the range intended to prevent firing the M2s over the highway adjacent to the impact area, and he’d instructed people not to use them. Their use was mandated in the range SOP, and frankly, if it had been me, I’d have probably shot his ass in a summary execution out there on the range if I’d been the guy finding it.

    Shit like that is why I say the Army is not truly an “armed service”. It pays a lot of lip service to the idea, but the fundamentals belie the verbiage.

    1. Tim, '80s Mech Guy

      Last platoon I was in had one headspace and timing gauge, one. If it hadn’t been a corporal’s key fob we wouldn’t have had that. We had zero extractors for separated cases, I fabricated something out of a cleaning rod with help from the file on my Swiss Champ. The “GOOD” barrels were kept safe in the conex so we took crap ones to the range forcing a quick class by me explaining why the tracers were blinking and going all over the place-last fifty range before Panama was straight APIT, in the daytime of course. The sixties were complete beaters of course but our supply/armorer kept them running, mostly, between spray painting the 1911s with dry lube so they LOOKED awesome.

      1. Kirk

        All of that shit is soluble by someone doing their fucking job and ordering the necessities. Somewhere, someone in your unit chain-of-command was not doing their fucking job, period. Even with the most egregiously tightened Class IX budget, there’s always an ability to get more funds for critical things like basic weapons accessories that help keep you from killing yourself or others running the damn things.

        Shit like that is why I meant to retire at precisely 20, before 9/11 happened. Nobody gave two fucks and a damn about issues of that nature, which is something I hear is creeping back into being.

        That noise you hear? That’s the audible disappointment wafting off the souls of every one of us who spent the entirety of the 1980s trying to unfuck the post-Vietnam Army. It’s like I was wasting my time and my life, ya know?

        1. Tim, '80s Mech Guy

          Fifth ID was a backwater, train to train, soup sandwich. We trained for EIB, CTT and SQTs-that last never happened in three years there. We trained for NTC, not to actually fight russians but to look good against the OPFOR. Rumor was our actual mission was to go to europe as replacements so unit cohesion was just not. Domino playing NCOs would occasionally check in to see if we were getting into trouble. It was the absolute worst place to send a new private or anyone who had been anywhere real. Good training is one thing and you can certainly argue those are all valuable skills but it was done so poorly after a while you realized you would do better to learn dominos.

  7. Kirk

    “It does not help that the standard M12 rack does not accept a rifle with optics. In the Arms Room, it’s still 1988.”

    Couple of things, here: The M12 is pretty much superseded. What’s being brought in is a commercial product that allows storage with optics, and a lot of other things besides. Google up “Spacesaver Weapons Racks”, and you’ll find examples of what I’m talking about. I’m not going to link, because that seems to trigger the spam trap here.

    A big part of the problem you’re highlighting here is derived from Army policy, and unit SOPs, which are so damn schizoid that it’s really impossible to completely describe to a layman. Hell, most Army guys don’t get it, especially the ones managing the weapons.

    Running an arms room is a full-time job. And, it needs to be done by someone with more than half a brain, because it’s that complicated. And, a great deal of the problem stems from the fact that the MTOE is basically schizoid by design–Let us say that we want to move a soldier from a line platoon to a new job as company clerk. In his old job, he was authorized an M4 with an optic, and a set of night vision devices to work with his weapon. So, in his new job? He’s not authorized anything past that M4, and the optic/NVG combo is platoon property, so it has to stay where it is. Cue either a clusterfuck of changing where that stuff is allocated, or a situation where you’re doing the ridiculous and having him re-zero a new weapon, which does just scads of good for training time and other issues. Hell, it is entirely possible for a unit to run through an entire year’s worth of allocation for ammo just accounting for all the nit-noid personnel assignment changes internal to the unit. I’ve seen that happen, believe it or not…

    Here’s what I believe: The weapons ought to be assigned and purchased with the mindset that every single man and woman in uniform is a meat-eating, front-line infantryman. Anything else is insanity, and having a situation where you have properly-equipped infantry doing all the fighting isn’t going to work, because as we’ve seen time and time again in both Afghanistan and Iraq, those elements are simply not going to be engaged by the enemy. You’ve got two types of units to chose from, as an insurgent: Meat-eating combat troops who have all the bells and whistles to make it easy to kill you, or the grass-eaters who don’t. Who do you think they’re going to pick? Yeah.

    So, you want to engage in modern war? Better get used to a couple of ideas. One, the linear battlefield is dead and gone. Not coming back, because nobody is that fucking stupid so as to engage us on one. Second idea is this: Every single person you put into uniform and deploy had better be ready to ruck up and take the fight to the enemy, each and every time the opportunity arises. If they’re not, replace them with civilian contractors, because they’re useless to the fight anyway. When you’re fighting these brush-fire wars, each time the enemy engages you is going to be one of the only limited occasions where you have them where you want them, out in the open and fighting. The rest of the time, they’re going to be playing civilian, invisible to the combat forces you’re exhausting running around looking for them. Why the fuck spend time “sweeping the countryside” for the insurgents, when they’re just going to melt into the background? Meanwhile, the opportunity to engage them is staring you right in the face, as they shoot up your convoys running logistics…

    Get engaged. Stop the convoy, engage right back, and then stay engaged until you’ve run the little darlings to ground and killed them, wreaking whatever havoc you need to in order to get to them. Break up a few civilian villages doing so? Tough shit–Next time, the civilians won’t want you doing that, so they’re going to tell the insurgents to get fucked, or warn you of their activities. When you make it abundantly clear that the price for engaging you is that you’re going to be run to ground and killed, no matter what the hell is in the way, you’re going to go through an educational process and quit doing that. What we’re doing right now is basically running a training seminar for the enemy, because we don’t do that whole “run to ground” thing. Shooting up an American convoy ought to be tantamount to a death sentence for you and everything around you, period.

    And, to make that policy work, every single combatant needs to be kitted up with the same gear on the same scales as the grunts, and their damn training needs to be upgraded. If their work is so distracting that this can’t be done, that MOS needs to be replaced by a damn civilian, anyway.

    I’d issue a kid his weapon for the rest of his career in basic, and then give him pride in ownership by letting him take it home after we swapped out the fire-control element with a civilian-legal version. Everyone ought to have their own basic weapon, and keep that throughout their careers, which would give them the incentive to take care of it, as well as a feeling of ownership. Basic sights, same-same–Organizational things like night-vision and thermals ought to be issued on the basis of need, and one damn thing I’d do is take a look at MTOE and say “OK, this headquarters element has enough joes in it to make a couple of line squads… Here’s where they break down, so this guy gets an M249, and that guy gets a grenade launcher, etc.”, until you have your set of HQ rifle squads that can serve as a reserve. If you’re going to plan to use people as reserve elements for your combat forces, you owe them the equipment to perform effectively in that role. Combat Engineers on paper as reserve Infantry? Then give them the fucking AT assets they need in order to do that job. Not giving them things like Javelin is utter bullshit, because when the day comes that those guys have to step up on the line and fight, they’re going to go down like nine-pins due to a lack of proper weapons.

    We cannot afford two classes of troops on the battlefield. If you’re in uniform, you ought to be a combat soldier, equipped and trained as such. Anything else is insanity, in this day of the non-linear battlefield.

    1. robroysimmons

      I really don’t think the American empire is salvageable, but maybe somewhere at sometime in the future there exists an America that is not broke and destitute and is involved in promoting freedom and democracy in some pesthole so perhaps your good ideas can be put to use.

      My experience as a weapons custodian in the USMC in a combat arms unit is that we were not that bad as described by the other posters. We could go to the field with M-2s with good barrels and head space them correctly (we even had them marked as to approx. head space).

      And you are correct about the officers, thank god for training that told them what end of the gun to point where.

      1. John Distai

        So Kirk, are you telling competent recruits to join the Marines? Every Marine a rifleman! At least that’s what they said on “Full Metal Jacket”.

        1. Kirk

          What I’m telling people is that the world needs to wake the fuck up, and start paying attention to what is really going on. Not even the Marines really buy into the “Every man a rifleman” thing, because they don’t equip everyone the same, or tell their support bubbas to do what needs doing when they make contacts. They talk about it, but they don’t live the credo the way we need to be doing it.

          I’ll say it again: If you’re in uniform, you need to be equipped and trained to the standard for Infantry, period. Every single support job ought to be broken down into modular squad and fire team elements, and be prepared to fight that way at the drop of the hat. If not, hire fucking civilians. The situation should be that your support job is the secondary MOS, and the combat infantry mission is your first, primary role. Aggression and preparation to take things to the hilt needs to be what we have going, and the insurgent or other opponent ought to be in such a state of fear before engaging us that he decides he would be better off shooting himself instead.

          These pansy-ass half-measures we’ve been following for decades need to stop. Even the Marines don’t do it right–They’re more right than the Army is, but that ain’t much of an accomplishment. The entire concept of “combat support” and “combat service support” needs to be thrown out the damn window, along with the idea that combat is possibly “somebody else’s problem”. When a support branch commander can get hit on a convoy, and even conceive of maintaining a career after blowing through it and running for it, we’re doing it wrong. That we even have officers who think that this is the right answer is a sign we’re T-totally fucked up in our training and indoctrination, and I’ve seen that happen in both Army and Marine units.

  8. Dyspeptic Gunsmith

    If you take a band new bolt and put it into a brand-new barrel extension, and you color up the back of the bolt lugs with some Dykem, then cycle the bolt a few times into the extension, you’ll see that even when brand new, not every lug engages. Sometimes, a majority of them don’t.

    Now, the error between the lugs that are engaging and those that aren’t might only be in the tenths of thousandths, but when they’re not engaging, they’re not taking up the force of the case pushing back on the bolt until the lugs that are engaged flex enough to allow the lugs without engagement to come into contact with the barrel extension. And given the hardness of bolt lugs (to gain strength), eventually, you can see some stress fractures on the lugs that do engage.

    Even if you start with something as pud-simple as a two-lug bolt, ala a Mauser (or any of the two-lug bolt actions), and you Dykem the back of the bolt lugs and then cycle the bolt against a spring force applied at the front of the action, you will often see bolt engagement that is sometimes much less than 50% of the surface area on the back of the lugs. Sometimes you see that one lug isn’t engaging at all.

    The way you get all the lugs to come into uniform contact is to use some lapping compound, a spring force on the bolt face, and some patience whilst you cycle the bolt a few dozen to few hundred times to get all the lugs lapped until they engage uniformly. The worst case I’ve had with a bolt action rifle required a pass in a Labounty bolt jig and a lathe to clean up the back of the bolt lugs (actually, one of the bolt lugs), then some lapping.

    This is why the Weatherby nine-lug bolts are a source of mirth among gunsmiths. Usually, one sees only three or four of those lugs making contact with the receiver. Despite advances in manufacturing in the last 50 years, there are some things that still require manual intervention with a set of skilled eyes inspecting along the way.

    1. Kirk

      Which is why you don’t want to be swapping bolts between barrels in actively used weapons–Each one has a unique pattern of wear that more-or-less means that the two parts are now essentially custom-fitted to each other.

      I used to show armorers that trick with the bolt lugs, picking two weapons at random and then swapping the bolts, using black marker as a substitute for Dye-chem. On the home weapon, the bolt would generally show engagement with the locking lugs when you closed it on a dummy cartridge. Put it into another weapon, and odds are that you’ll find lugs that aren’t engaging. Cue enlightenment…

    2. robroysimmons

      We Weatherby owners are a faithful bunch, so please keep it quiet. Mine is a 6 lug .308, (glory to all things Roy W.) and it is sweet save for that damn ball detent for the bolt and the fact it won’t reliably fire NATO ammo.

  9. 10x25mm

    A couple of engineering and metallurgical points:

    It is almost impossible to machine multilug systems to an accuracy which gets all bolt lugs in bearing contact with their counterparts in the barrel collar or receiver. A quick prussian blue or lithium soap run will confirm this. In AR’s, typically only two or three lugs make positive contact out of the box, the rest come into contact after some wear on the first contactors.

    Weatherby gets their Mark V multilugs into contact by touching off an extra high pressure proof round before stocking and full assembly, then rechecking headspace. Even so, they are happy when six lugs come into bearing contact.

    Think GI AR bolts are still made from carburized AISI P6 tool steel (Carpenter 158), a steel not known for its plastic deformation. If only one lug is bearing out of the box, that lug will be the only one bearing until it fails. Unless there are some very low lash lug sets and the contacting lug wears against its partner quickly.

    It takes two to tango. If you shear a bolt lug as in this instance, you probably have a damaged partner (mating) lug in the barrel collar. Both the bolt and the collar will need replacement after this kind of event. Geometrically, the collar lug is stronger than its partner bolt lug, but not by that much. The geometric strength difference does, however, explain why bolt lug failures are far more common than barrel collar lug failures.

    Fatigue fracture fractography at low magnification is generally very smooth in appearance, at higher magnifications a ‘beach mark’ appearance becomes apparent. These beach marks represent the progression of the crack with each subsequent loading cycle. The final fast fracture in AISI P6 occurs when the remaining steel lacks adequate strength to resist a single cycle. This occurs by cleavage, whose fractography is granular. Cleavage is a near zero ductility event which changes direction across each grain to take advantage of the lowest resistance path available across each grain, This creates a faceted fractographic topography which appears granular at low magnification.

    Overall, absence of trauma subsequent to this event is a real tribute to the design margin in the AR design. Imagine the consequences of the failure in a single lug design, such as an FAL.

  10. Frogdaddy

    Are we saying I can’t have a spare carrier group from same manufacturer of rifle? Somewhere down the road the original may have issues, I can’t just plop the new carrier group/bolt into this M4 ? If let’s say I’m in the field during a grid down situation, what tools will I need then if “just” swapping won’t do.

    This civilian needs clarification.
    Thanks.

    1. Kirk

      Weapons wear. That’s a fact of life.

      How they wear, and what parts take that wear isn’t predictable. Same manufacturer bolts may be completely interchangeable with the receiver extensions on the day they leave the factory, but add in X number years of wear, and all of a sudden, that interchangeability goes to shit. This is why you have to use the gauges religiously, whenever you change the bolt/barrel on these weapons. You may get away with it a hundred times, but the hundred and first…?

      You don’t notice this as a civilian with one gun. You don’t make these swaps a hundred times, the way a military unit would. When it catches up with you, however, the results can be catastrophic. Which is why I won’t take the risk. A set of gauges from Brownell’s is cheap enough insurance.

    2. Miles

      Spend the $ and, at least, buy a set of headspace gages.
      Midway, Brownells, Graf & Sons, Natchez and God knows who else has them.
      They’re not that expensive, so you can buy several sets

      1. Dyspeptic Gunsmith

        Manson Reamers has sets of “rough and ready” gages. These have all the dimensions correct, but they lack a ground polish on areas of the reamers where such a finish isn’t necessary, so they’re quite inexpensive – $60 for a set of three gages (go, no-go and field) in very common chamberings: 5.45×39, .223 Rem, .308, .30-06 and 7.62×39.

        1. Frogdaddy

          And so, by checking this prior to any departure on my way to use my M4, any sign of stress or wear evidence shown by the use of the gauges, warrants a swap of barrel?
          Thanks.

  11. Roger V. Tranfaglia

    WOW
    Never been in the service, but I did learn something today……

  12. Docduracoat

    Frog daddy,
    See my first post in this thread, it explains the entire thing.
    I have been a gun nut for 20 years and always considered a bolt a wear item.
    I shoot my guns a lot and my AR bolts usually break at the cam pin hole or lose a lug after several thousand rounds. I have always just bought a new bolt and put it in the old carrier.
    Now I know I need a ” field ” gauge to be certain the gun will work properly with a new bolt in a used gun

    1. Frogdaddy

      Nice read even for an amateur like myself. My M4 is not chrome lined from when I built it. And if in the field and you have this type of failure and the gauge indicates scrapping the barrel, you’re hosed or dead in the water in midst of battle? Is that what I’m hearing/reading? Or do I lug (no pun) around all sorts of spare parts including a spare barrel and nut wrench? I understand failure is the nature of the beast, but are some parts more durable than others (chrome lined vs. non-chrome) ? Can proper maintenance reduce the failure rate or increase the longevity of these parts? Sounds like gauges are a necessity for the non-gunsmith as well.
      Thanks Doc.

  13. Tennessee Budd

    In 2008, I decided to go back into the military, Army this time (I was USN for 1 enlistment). I thought armorer, or “armament repairer” ( I think it was 45K) , would be good; I like weapons, have machine-shop experience, etc. Once I found out that it was in Supply, however, I backpedaled. The Navy taught me to be that smart, at least. Decided I’d rather go 19D, Cav Scout.
    Turned out to be irrelevant, as events transpired. 90 days before departure for the WTC (Warrior Transition Course–back then, they didn’t think redoing Basic was necessary, however much I disagreed), a truck left-turned in front of my Honda, altering my plans, not to mention my body. They don’t want you if you can’t run, & sure don’t want you if you’re in a wheelchair. (I got out of the chair, eventually, & can walk in a fashion now. Still ride, too.)

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