M4 barrel fell off


“FREEFALLE7”, a Ranger Instructor, posted the above photo at AR15.com. A student, on arrival at the Swamp phase of Ranger School, showed him his M4 and said “my barrel fell off”. His punishment for not telling an instructor at the Mountain phase (the previous phase) about the broken M4 was to be given a M240, which weights 27 lbs, as a replacement.

Keith J came up with a plausible theory of how this happened

Salt water in the threads. Steel rusts. Steel in contact with aluminum. Aluminum then oxidizes, reducing the rust to iron dust. Joint remains tight until it is abused, then it just falls apart.

This all started when the barrel was installed and it was tightened a bit too much, causing the anodizing to crack

Of course this would have happened over a long period of time and use.

Thanks to Jay for the link.

Steve Johnson

Founder and Dictator-In-Chief of TFB. A passionate gun owner, a shooting enthusiast and totally tacti-uncool. Favorite first date location: any gun range. Steve can be contacted here.


  • Crystal

    “His punishment for not letting an instructor at the Mountain phase was to be given an M240 to carrying around instead of his carbine.”

    Didn’t quite understand this part… Rephrase? 🙂

    • Crystal, lol, that teaches me not to forget to proof read. I hope is is clearer now.

  • bad, bad memories…….

    “troop, you know what you and an aluminum can have in common? you can both be recycled!”….arghhhhh!

  • Matt Groom

    Holy full-auto, Batman! That’s one symptom of some serious, sustained, full-auto fire! And more than once! He’s lucky he wasn’t firing that thing when it gave way!

    The only logical reason I ever heard for the military’s adoption of three-round burst was that it would slightly decrease the rate of fire when using the newer 30 round magazines. Apparently, the M-16A1 had a tendency to launch their barrels when sustained FA was used in training, with total disregard for the amount of heat that was going into the receiver. There’s no amount of training that can counter a reckless disregard for safety, and firing a rifle until the barrel is cherry-red is something I’ve seen and heard about people doing on more than one occasion. Respect your guns people!

  • jdun1911

    Keith J know his stuff. Another thing I would like to point out is that the armorer probably forgotten to grease the threads.

  • Dom

    Not being a military person familiar with the practices of military armorers, I have to ask – what is there in place to protect a solider from having this happen to his combat weapon?

    And since there is often such rousing discussion re: weapon failures on this blog, what kinds of horrible consequences could there have been if the rifle had come apart this way during firing?

    I’m thinking unsupported chamber explosion and a wild shot.

  • whatever

    If I had to guess, I’d guess that the barrel nut wasn’t torqued down properly and so worked itself loose through vibration and heating/cooling cycles.

    Weird question I’ve had for a long time: Anyone make a steel upper for the AR? I know that there’s basically zero point for having one made out of steel rather than aluminum but I just thought it’d be neat.

  • jdun1911

    Dom, it is up to the solider to make sure his/her equipments works.

    These weapons are not safe queen. They are used and abused regularly by kids. Not only that these weapons are suppose to last forever. They are repair or retrofit at the armory level. There are some first generation A1 lowers and uppers that are still being use by our military.

  • jdun1911

    The gas tube prevent the barrel nut from twisting/working it way lose.

  • ?

    One more reason to switch to AK

  • Edouard

    Yet one more reason why Armalites=epic fail

  • Mu

    I’m amazed that this isn’t a regular occurrence, aluminum threads with steel nut requires a very carefully torque to get both good connection and no damaged threads. If you add aluminum-typical fatigue and creep, you can probably see this even in weapons that were always handled according to specs. With this weapon obviously in a training unit and subject to both heavy firing and rough handling (after all, the trainee knows he’s getting rid of it at the end of his class, and he doesn’t have to rely on it in combat) it looks like they got a lot of use from this upper. Would be interesting to find out when it was made.

  • jdun1911

    AK, give me a break. If you think those rifles doesn’t have their own set of problems I got a bridge to sell you in NYC.

    The upper probably got rebarreled many times. The upper as well as the lower receiver should last forever. The problem was over torqued and most likely the armorer didn’t grease the threads to prevent corrosion.

  • ?

    You want a break? Sit down and enjoy 🙂 Everything and anything has some problems, but compare to M/AR rifles AK looks pretty good. It is so sad that our guys still has to do there job with weapons which might be more dangerous for operator then for target.

  • jdun1911

    You have no idea what you’re talking about. Go back and play your video games. Let the grown up do their stuff.

  • “He’s lucky he wasn’t firing that thing when it gave way!”

    I don’t think the consequences would be as bad as you think. The bolt would be locked into it during firing, and the tube stopping rotation… actually, and I’d have to look at one real closely, in battery it’d be hard to impossible to launch the barrel, some friction point in the loading/bolt return phase would have to shove it off.

    At that point, with the barrel going/gone, the front of the reciever is open. I don’t think you’d have enough instant pressure do get frags or gas anywhere but out the front and out the ejection port. Then again, the cartridge would be hanging halfway out the front of the reciever… but then AGAIN, that also means it has no cross-axis reference, the ejector would toss it off to the side before the firing pin could strike it. THEN again, if the ejector’s not working right if could be held to the bolt face tight enough to fire… but THEN AGAIN (heh) the gun would have tied up before the failure point.

    Look at the photo, the threads on the reciever are noticeable flattened. I bet the barrel nut was corroded (as above) and/or was out of spec and overtightened at some point, predamaging the threads to where they stripped later.

    It’d take some force still to strip those threads off… I’d LOVE to know at exactly what point in training it broke loose. You can’t apply side force to the barrel nut because the chamber extension is tight in the receiver… you can’t push on it for the same reason. It’s have to be pulled, or have some action applied that yanked the barrel straight off.

    Do they still use bullet traps or blanks in simulated fire training, or is it all IR now?

  • You know, the above thoughtline is why I respect the AR15 platform. There’s a hell of a lot of engineering that went into that gun to make is as simple and “foolproof” (engineering wise) as possible.

    I was considering the same the other day on different bounds, and the only things I could come up with as improvements were a different mounting method for the stock tube (no big-arse threads, interference pins, I’m mentioned this before), a directly-attached “post” operating handle instead of the one-way top mount it has now (though, would really slow the lefties down), and modular/replaceable drop-in fire control assemblies, like Chip Mc makes but containing the FA sear or 3-shot cam.

    Since I’ve never seen some of the above, I guess that’s two patent-disqualifying posts of the day…

  • Jeremy

    My guess would be that the weapon had been rebarreled several times. Its also possible some Gung Ho troop went a little further then he should have in disassembly for cleaning. As a unit armorer I had to repair several M-16’s that people took apart when they should not have.
    I love the M-16/AR-15 platform. Anyone who visits my house will see that. I built my first AR in 1988. When the kit arrived, nothing was assembled, I repeat, NOTHING. The instructions warned that barrel torque was critical. Everything since has come with the upper assembled. Manufacturers are doing this to eliminate risk from people who don’t torque it right.
    Cleaning the chamber is much easier with out a pesky upper in the way, but I’ll be darned if I’m gonna do it that way.

  • ?

    jdun1911, nicely said. Not everybody here around is a little kid. I do have some kind of limited experience with m16/Ar15 platform. I served with SVD. We study M-rifles and it is wonderful, over-engineered rifle. Very pleasant to shoot at shooting range. Complicated enough that it require an specialist to do half of the adjustments, several day of training for soldier to clean and extremely dangerous to perform full maintenance at the field. American pride and political games preventing soldiers from getting a good weapons which plenty around.
    Give M-rifle to police and SWATs, give soldiers combat weapon.

    • David/Sharpie

      You say it’s hard to disassemble the AR-15

  • Meltron

    Question Mark,

    Any TRAINED soldier can handle his rifle, and AKs while very reliable suffer in accuracy and modularity. Frankly the ”real tough guys” in the military community like SAS and Rangers prefer an M16 carbine because it can be customized much more easily than and AK, and of course its accuracy.

  • Bobby

    This is why improper maintenance = fail.

  • Asclepius

    Why is there a string tied between the carry handle and the front sight post? Could it have been tied there as a quick fix by someone who noticed their barrel was wiggling around?

    If that is the case, then there were warning signs that this rifle should have been taken out of service and repaired before this catastrophic failure.


  • AK 47 can be modified just as nicely as the AR15, provided you have the parts. But 7.62 AKs simply cannot match the accuracy of an AR, nor its highly superior ergonomics, which make it easier to shoot and shoot well.

    For ? — Anyone can clean an AR 15 — I’ve seen 12 year olds do it.

    The AK is inferior in every way but 2. Reliability and power.
    And I chose an AK over the AR15 for my personal use.
    We need a new rifle, not a pirated Soviet design from the 1940s.

  • T>L>

    Looking at the photo the gas tube appears to be intact. The tube goes through “slots” in the barrel nut. So this pictures tells me the barrel nut did not loosen but there was a catastrophic failure of the thread on the receiver. There was obviously some serious abuse going on here. I can tell you from experience when men are training to the very ragged edge of exhaustion such as certain types of Secial Ops training the first thing they do is neglect their equipment. Not pointing fingers or putting blame on any one but I’ve seen guys that could destroy an anvil with a feather.

  • Rick Randall

    Asclepius —

    That dummy cord is there so that when the carrying handle decides to do a little independant night recon of the Florida swamps, Young Ranger Candidate won’t lose it.

    EVERYTHING gets dummy corded to something else. I was in one school where they had us dummy cord the rifle AND our webbing to our bodies. (I failed to do so, and dummy corded my rifle to my webbing. I’ve read the reports on teh D-Day landings — if I decide to part company with my web gear, I generally have a damned good reason. Of course, outside that one course, the only time I dummy corded the weapon was when doing something involving water deeper than my waist. The rest of the time, I just used the sling with the patrol adaptor.)

  • zach

    I don’t want to start another AK vs. AR debate, but it’s really usage and personal preference. Granted I would rather use an M4A1 with full RIS then an old AK-47, but they both have there good things and bad things (which I won’t list cause it’ll take too long). But seriously outlining every flaw and advantage of the two weapons has gotten REALLY old

  • zach

    I’ve also never seen that sort of thing happen before, and it makes sense now why they pretty much only ship uppers with the barrels pre installed now.

  • Destroyer

    nice…i remember the swamp phase…or what id like to refer as “the most miserable time of my life”.

  • Hotgun

    A clear photo would really help determine what happened.
    I tend to disagree that the threads stripped or corroded. The metal components are well treated / protected against corrosion. Also, there is an anti-sieze lubricant on the threads (heavy grease).

    Illustrates another great use for paracord.

  • JoeB

    Well, did they ever think of the possible senario that the M4 is a peice of crap?

  • Rick Randall


    How, exactly, are ANY of the supposed problems of the Stoner AR15 system going to result in this failure?

    Direct impingement gas system? Nope. Has NO effect on the barrel nut, except the barrel nut isn’t subjected to as much stress from a moving piston system could potentially give it via vibration.

    Magazine? Um, no.


    Because that’s all there is in terms of mechicanical reliability to question the AR15/M16/M4 on. Neither of which has any bearing on the problem at hand.

    UNLIKE the FAL, the AR15/M16/M4 barrel nut isn;t really critical to keep the gun headspaced, nor is it subjected to a lot of firing stress. Which is rather like the AKM, only the M4 barrel nut is more solidly built.

    The barrel retention method of the AR system (that would be the thing that failed here) is actually pretty common in modern arms, and is a VAST improvement over earlier methods, especially when it comes time to rebarrel a rifle. Because of teh method used to assemble the barrel and upper receiver, the barrels can be headspace at the factory, and the armorer on deployment DOES NOT NEED TO SET HEADSPACE BY DRILLING. He inserts the barrel into the receiver, torques the barrel nut appropriately, and slides the gas tube through the hole. All he needs is a wrench with no moving parts (and his also is used to stick the flash suppressor on), a way to hold the upper receiver steady (i.e., a vice and a holding bracket), goop so the threads don’t sieze, and a torque wrench to knpow he’s using the proper torque.

    He doesn’t even need a headspace gauge for rebarrelling, although he will, just to be sure. Hell, the Army only uses ONE headspace gauge for the M16 — it’s the FIELD gauge; if it fits, the gun needs a new bolt.

    The factory headspace on the barrel is so consistant that ANY in-spec bolt (including those with in service wear) is safe with ANY in-spec barrel assembly (including those with in service wear), and ANY new in-spec bolt is AUTOMATICALLY correct for ANY new in-spec barrel assembly.

    Let me know the next time you’ve rebarrelled an AK47 or G3 and rest the headspace. Let me know how you replaced the barrel and adjusted the headspace on an FAL without a bin full of locking shoulders of various sizes, to ensure you have the correct headspace.

    Then explaine to me by what magical force their barrel attachment points are less likely to fail under the same basic circumstances.

    I’m guessing you’ve probably never actually SEEN a real AKM, especially one that’s been treated like crap for years. Ever seen a rivet sheer from internal flaws? Know what happens when the rivets? Guess what, sparky — you get the same exact result as this M4 — the friggin’ barrel falls off. How much cross section is there in the AK front trunnion rivets? How much cross section is available in the M4 barrel nut thread? What kind of quality control goes into making an AK rivet? How about that blued steel sheet steel those rivets are holding to the trunnion?

  • IRgRunt

    Held on by 550 cord, beautiful. Somehow, when the instructors told the students to “Dummy Cord” their serialized gear, I don’t imagine this is what they had in mind….I don’t envy him having to run a FTX with a 240…

  • Sid

    This is a school weapon. It has seen more battles than the weapons on the real battlefield. If it had a round counter on it, eyes would water just reading it. US Army inventory policy is to cycle the older weapons to the schools.

    This is not an AR v. AK issue. This is not really a maintenance vs. operator issue. This is an over-used weapon in repeatedly placed in the hands of overly tired and sleep-deprived Ranger students. I work as construction project manager now with a staff of 30 sharing 3 vehicles on post. The worse the AC sounds, the louder the radio will be playing when you get in. It is always the next users problem and is only to blame on the user when it finally breaks.

  • Darian

    @ Asclepius… The string was obviously tied there AFTER the failure as a way to ensure both pieces of the weapon were kept together..

  • The armorer fucks up and the punish the grunt. Go figure. This is how the military loses so much talent to the civilian andprivate contracting areanas. Better pay and a small fraction of the assinine bullshit.