The Burned Out Barrel Problem

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

Let Us Propose a  Way of Viewing Malfunctions

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

Malfunction Matrix

Malfunction Repairability


Irreparable / Depot Org Repair Field Repair

Class A (total dysfunction)




Class B (serious degradation)




Class C (mild malfunction) C1 C2


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

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

Bolt Failure in the M4

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




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


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

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

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

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

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

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



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

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


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



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

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

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

What ties these problems together?

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

So What’s the Answer?

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

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

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

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

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

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

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

And that all starts with a #2 pencil….


16 thoughts on “The Burned Out Barrel Problem

  1. RobRoySimmons

    Accuracy I do believe can be unduly affected by the crown condition, and from my days with issue weapons we all but took a hammer to the crowns on our A1s and 2s. So your test of using accuracy could seriously be affected by that.

    Also could your perspective coming from the high speed side of .mil be causing you to overlook what the maintenance on the lower speed side is like in .mil? My guess is that you fellas could actually put copious amounts of lube on your rifles whereas in my time in the shoe shine military of the early 80s it was light coat of lube and of course shiny crowns. We never got the memo to lube the snot out of the bolt on our 16s and I worked as a weapon’s custodian. Which reminds me I should look in my LMT’s operator manual and see what they say about operation and lube amounts, I would bet it is no better than the info I had back in the day.

    1. Hognose Post author

      The crown of the M16 series weapons is protected inside the flash hider. You can screw up accuracy on the old prong (pre-’66) type if you really Bubba it up, but the cage type suppressors, their ring ends can take a beating without much effect on accuracy. We did not notice any significant POI change or accuracy change when we changed out the A2 type suppressors on our M4A1s for the KAC flash suppressor/sound suppressor QD mount unit.

      1. Dyspeptic Gunsmith

        You can mess up the crown of a M16/AR-15 barrel with a jointed cleaning rod if you stuff the jointed rod into the muzzle without a rod guide. When the rod bends as you’re pushing a jag/patch or brush into the bore and a joint catches the crown… there ya go.

        Same deal on a M14/M1A.

        I recommend cleaning AR-15/M-16’s from the breech end, using a bore guide and a one-piece rod. On all rifles, I recommend a one-piece rod. Those jointed steel rods that are supplied in GI-style buttstock kits – they’re a great field expedient, but they’re the very devil on barrel features if a joint catches the crown or the throat. Next thing I recommend on all rifles is using a bore guide so keep the rod straight in the bore and not allow a flexing rod to rub unevenly on one side of the bore.

        If you have a really nice barrel, don’t pull the brush or jag back into the bore after you exit the muzzle. Remove the jag or brush, pull the rod back, re-attach the jag or brush and start again from the breech. This is how benchrest and F-class shooters treat their barrels, and they’re zealots at protecting their crowns. Of course on the M14, it isn’t possible to clean from the breech, so I will put a bore guide on the flash hider, put the one-piece rod down the bore until it comes out in the chamber area, attach my brush there, and pull the brush from the chamber to the muzzle.

  2. Daniel E. Watters

    “Attention is respectfully invited to Captain Leefe’s suggestion of a metal oil bottle. The magazine rifle is a small machine demanding more intelligent care from the soldier than the Springfield rifle. Certain working parts must be kept lightly oiled. The old idea that the soldier’s piece must not show, at inspection, any signs of oil must give away to the necessity for keeping these parts lubricated.” – Lt. John T. Thompson, US Army Ordnance Corps (1896)

    Nearly 90 years later, then Major David Lutz (USMC) inserted a page into the original M16A2 manual advising NCOs to give up the white glove approach to inspecting the rifle as it needed to be properly lubricated. This page was blanked out in subsequent editions.

      1. RobRoySimmons

        I didn’t want to beat the dead horse about the treatment we gave our weapons, but I just have to say we in the lower speed combat arms were at time medieval so as to attain the lowest common denominator of military service. I probably read that manual when the MC issued us our A2s which compared to our ragged Vietnam used A1s were gifts from the gods.

  3. Big Country

    Hognose: Great article! I’m the Small Arms Repairman (Gunsmith 3) for The US Army Reserve here in sunny Florida. One point I see that you overlooked in detail but mentioned in passing I’ll throw out there is one of the more common failures I see during a TI/Gauging. This being worn/leaking gas tubes and or damaged BCG Gas keys.

    B/C I’m dealing with REMF units, the weapons either are NIB NRA Grade “Fine” OR they look like someone dragged them down Route Irish lashed to the back of a LMTV. Either way, all humor aside, the worn barrels we see are few and far between, but the bolts are usually pretty thrashed, to include gas rings (50% failure rate) and gas tubes and keys being the next most common issue. (about 25%). The failure is the wear on the tip of the interior of the gas tube itself, which should have a very slight ‘mushroom’ head shape in order to allow a tight fit into the gas key on the BCG.

    Although not in the 23&P Officially, my partner and I bought the Gas Tube Gage from Brownells and it does a pretty fair job of estimating wear on the interior part of the gas tube. If it IS worn, the gauge slides all the way down onto the gas tube (inside where the charging handle rides) and free spins. If the tube is G-t-G it stops as soon as contact is made and won’t go any further in. Mark One Mod One Eyeballing is needed to insure this.

    The Gas Key itself of the BCG is usually damaged (chipped and worn) primarily as “Joe” tends to drop his BCG during cleaning, and more often than not, it lands on the gas key lip… We have a tool to try and re-round and reshape it, but more often than not we replace and restake it as we keep a pretty large number of this part onhand despite regs.

    A poor seal on the Gas Tube and Gas Key means failure to cycle, failure to feed and generally can be a pain in the ass b/c a regular “Joe” wouldn’t normally be able to examine this (the gas tube that is) without the specialization training that I have. A damaged gas key however CAN be eyeballed, but a lot of “Joes” would rather not admit he/she fucked up their weapon and face a ILOD Investigation…

    Otherwise… GREAT article on the Black Rifle… Keep up the good work man and email me… the offer of that Knight 1st Gen front grip is open for your build…

    1. Hognose Post author

      Great stuff, and maybe you can get that gage taken up officially and spare some other armorers’ time.

      I imagine that the gas tube/key problems manifest like all gas system probs in the AR do, short-stroking with all the various FTFeeds and FTEjects that come from that.

      In 1994, DA/DOD did a shufffle-change-o and put all the combat arms in the ARNG and all the CS and CSS (aka REMFs) in the USAR. Reason was, active forces needed some of those CS and CSS to go to war, and some governors were trying to run their own foreign policy. It was also good cover to eliminate 1/3 or so of reserve component end strength.

  4. WCOG

    Fascinating stuff! As a reliability engineer in the aerospace industry I generally find gathering in-service data from operators even on something that produces as many documents as aircraft maintenance to be more difficult than pulling teeth. Instrumenting in-service weapons with accelerometers is a great workaround, although I wonder how much pushback they will get/have gotten from operational units that don’t feel like carrying around instrumented weapons. Do you have any kind of update on whether this effort went forward or not? From an internet search it looks like they let out a contract for the counter units in 2010 but I see nothing else since then.

  5. Dyspeptic Gunsmith

    The AMU has developed quite a bit of information on barrel life. At their level of competition, I think the barrel life is much shorter than combat-acceptable accuracy. I’ll ask one of my gunsmith buddies who was an AMU trigger-puller what his expected barrel life on an AR-15/M-16 was.

    One interesting thing AMU gunsmiths have told me is that the scrupulous logging of every round sent downrange allows them to spot when the barrel is about to show accuracy-affecting throat erosion. The groups actually tighten up from the early barrel life groups for 100 to 200 rounds before starting to open up as you’d expect from a shot-out barrel.

    AMU members have told me of having their gunsmith come to them and say “Your rifle has to come into the shop – your barrel is about worn out…” and the shootist says “Like hell! I’m shooting better groups than I have in months!” The gunsmiths sometimes have to pry the rifles out of the hands of the shooters to replace the barrels, but they have great mounds of data to show that they know what is coming.

    Barrel life in rifles in general is linked to how “overbore” the cartridge is. Highly overbore cartridges (eg, the 6.5-.284, .220 Swift, 7mm Rem Mag, .25-05, .22-250 AI or even the straight .284 Winchester) are considerably overbore, and they burn up barrel throats in as little as 1200 to 1700 rounds.

    The .223/5.56 NATO round is much, much less overbore than the above cartridges, and I’d reckon that a good quality barrel treated well (ie, not overheating it with lots of rock-n-roll firing), should last at last 3,000 to 4,000 rounds with good accuracy.

    1. Hognose Post author

      Fascinating stuff. One thing the AMU is supposed to do is feed information back into the operational Army. That doesn’t always go well, I think the fault’s on both sides. Many combat guys hear “AMU” and think “irrelevant gravelbellies.” They’re wrong; the science of the target shooter is valuable to anyone who prizes hits, and nobody should prize hits more than guys who are trying to hit the folks trying to hit them.

  6. Stefan van der Borght

    Firstly, what do the pro’s think of boresnakes……since they’re only pulled out muzzle end, are these things better per se than cleaning rods?

    Also, what about more intensive training in making every shot count? I remember being taken severely to task for a follow-up shot on a reaction range, on the FAL; where one only had 20 max in the mag, 120 as standard field issue, and institutional memories of D-Coy at Long Tan where customers just kept coming. But, each one of those shots also made a satisfyingly large hole. I also remember later in my private shooting, where I actually learned how expensive it was to blat off full bore ammo (also learning the value difference between old milsurp .303, new factory loads, and good handloads). Make every one count….since the first one is so important, because it also acts as a beacon. The suppression at Kohler should have been handled by the crew-served stuff…..but, that was mostly pegged by the opposition on their first play. What was the training like for such a situation….improvise at least concealment, even if cover was impossible? Also, the doctrine of keeping the MG as primary firepower, á la Wehrmacht, and the riflemen as support? Hmm. Ok, I’m less than nobody in this accumulation of pro’s, but I’d like to learn, and also perhaps others….

    1. Hognose Post author

      I like the ones that are a woven soft fiber, but not the ones that are a chain like the old Wehrmacht version.

      1. Dyspeptic Gunsmith


        I’d say that the #1 thing I’d advise WRT snakes is the same thing I say about bore brushes in general: Pull/push the brush OUT of the muzzle. Don’t pull the bristles into the muzzle, over the crown.

    2. Miles

      1 Very nice ppt Hognose. I’d seen some individual slides, but never the whole presentation.

      2 “Firstly, what do the pro’s think of boresnakes……?”
      The ones I know use the Hoppe’s version as a standard item for all calibers. However, the sectioned rods are always at hand in case something gets stuck in the bore.


      I have been shooting a lonnng time and the most of it early on was with Match rifles, BR and High Power ( service rifle etc) and in those times a man has a fetish for proper and careful cleaning that is effective. I learned these lessons and they have kept me in very good shape and I stick to them.

      I think the pull through cleaning systems made of cloth are just as dangerous to the bore as a segmented cleaning rod

      what most do not realize, is the cloth picks up grit , grime. tiny pieces of metal and particulate that acts to score the inside of the bore. And there if the danger of them binding and getting stuck in the barrel. A one piece rod with a boreguide of the proper size with the rod the right length. If the rod is too long its weight at the end will droop and wear on the crown as it is moved back and forth. A coated road is safer but must be wiped clean after every backward stroke through the bore. Proper fitting jags and bronze brushes with the metal end ground off or dipped in rubber to protect the bore. Always spray the brushes off with something like brake parts cleaner before using. otherwise you are just shiiting up the bore again. Also never dip the brush in the solvent or you will contaminate it. And drip the bore cleaner onto the patch. Dont dip brushes and patches in the cleaner. or you are going to contaminate the solvent and send it right back through. I would stay as far away from a bore snake as I could get. same as the segmented rods.
      This is the points I was taught by my ancient mentors who spent much time in the NBRSA and High Power taking care of match bores, Its not THE way. but it is indeed very good advice.

      another suggestion is not to waste money on Hoppes 9. the nitro-benzo that made it effective up until the 70s was banned by the EPA and is not longer in the stuff. Like New Coke. its not the same classic formula anymore. Its not good for much real cleaning anymore if you want match level clean. I suggest TM Solutions, Butches Bore Shine and to a little lesser degree, Shooter Choice, JB bore paste for very bad copper and corbon fouling .

      As an IFY. Colt armorers and quite a few other have long said that there is not need to stagger the gaps on the gas rings. The guns will run on just one ring if its in good shape . I have tested this myself many time. I believe it was even Dean Caputo who made the point that the gaps dont stay staggered longer than a few shots anyway. I guess its tradition to stagger them. But I respectfully submit to not worry about it anymore .

      Those are my thoughts on Cleaning and you can find greater detail over on our website if any one cares to look I will say those tips are for match guns, or anything you want to really treat well for accuracy sake. I do not clean fighting guns to that standard. Im more of the wipe and scrub the BCG and inspect it and get it as wet as a golder digger seeing a 100 dollar bill, punch the bore a few times, clean the chamber and lugs a bit and soak it all down copiously with oil . I have not had a malfunction in carbine with up 14,000 rounds in 6 month when just using that method, those are ( for me)hard work guns though of course.

      But that is all like, just my opinion man.

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