Colt M16A2 Commando KABOOM

TFB’s resident South American gun expert Ronaldo Olive sent us these photos of a Colt M16A2 Commando (Model RO735B) owned by the  Polícia Militar do Estado do Rio de Janeiro (the Rio de Janeiro State Military Police).


The upper receiver has been blown apart, the magazine baseplate has been blown off and the bolt is lodged in the buffer tube. These symptoms point towards an out of battery detonation. I hate to think what damage was done to the operator. Ronaldo was not given any additional information about the incident.






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.


  • That uh… I bet that hurt like Hell. and if not that, I bet the operator at least didn’t have to pee afterwards.

  • rk

    stuck firing pin. out of battery detention would be my guess.

    • Impossible on an M-16 series. The design precludes it. The firing pin cannot reach the primer until the bolt is fully closed.

  • Mr. Fahrenheit

    That’ll buff right out.

  • Cdn Phil

    That is very cool. I was involved in investigating the blowing up of some Canadian made guns during my time as an engineer at Diemaco (a couple were intentional!) and I can tell you you got a spectacular one there.

    A couple of observations: without either monkeying with the system or doing something deliberate, it is VERY difficult to get an out of battery initiation in an M16 design due to the design of the cam pin track in the bolt/carrier requiring the bolt to be in battery and rotated fully before the carrier comes forward in a straight line on the last part of the counter recoil stroke. This last movement (about 0.060″) is also designed to be the movement that allows the firing pin to come into contact with the primer on hammer fall. If the bolt isn’t rotated by the cam pin the carrier movement forward stops and the firing pin can not reach the primer. Firing out of battery is probably less likely than an in battery discharge with a very significant bore obstruction not too far down the barrel OR you had a super hot load, probably hotter than you could do with a case full of Bullseye though I can’t say as I’ve never been allowed to try that. (I’d like to!) You are likely in the 100,000 psi plus range to get this kind of damage and likely over 125,000 -> 150,000. We fired proof rounds that regularly got to 90,000+ in the chamber (we measured them) and our C7s would eat those up all day and function fine so this was a spectacularly high pressure event to split things like that.

    (As an aside on in vs out of battery, if you recall the carrier bounce issues of the M16 on auto in the 60s, these failures to fire were caused by that last 0.060″ bounce back happening as the auto sear released the hammer but before the hammer could fall the carrier literally bounced back off the barrel extension coming straight back and when the hammer hit the firing pin it was prevented from protruding out the bolt and contacting the primer. This led to huge experiments with floating buffer weights to reliably solve.)

    The removal of the extractor at high speed is typically expected as is the blow out of the magazine. Also the splitting of the upper is common in such events (though this one is on the spectacular end of the spectrum for certain). Splitting the carrier body is also indicative of a superb event, I’ve split them but not to the extent of fracturing off large chunks on the sides like this event. The bottom of the carrier between the deeper cuts for the magazine lip clearance usually is the path of the split there, but this one did that and blew out the right side. Nice. The key being intact says not likely did much gas blow through the gas tube.

    Its interesting that the bolt actually was driven rearward and rotated clockwise rather than counterclockwise as it normally would. If the obstruction was far enough to allow the gas tube to fill and/or it was SO violent that even though the obstruction was likely blown out, it seems it was already past the time that the action had started to loose integrity. The other culprit might be someone playing with the bits and moving them after the fact as it should have been in lock on the left side.

    Epically cool series of pictures. Do you have anything looking down the bore or showing if there is anything left of the cartridge case in the chamber or welded to the bolt face? Also interesting it is an A2 Commando and it doesn’t look to have US Govt markings (if so it wasn’t sold FMS…) Too bad they really did a job on the serial number!


    • Thomas Gomez

      Awesome knowledge transfer Cdn Phil. Did you ever fire some “half loaded rounds”? For example the cartridge was half filled while perpendicular but when placed in a horizontal position (in a magazine) the powder spread out so that it could all be ignited at the same time? High PSI?

    • FourString

      Whoa a Diemaco engineer spotted in the wild. Crikey!

    • Diemaco… it.

      Sending my recently graduated resume tomorrow morning.



    • There’s also no telling who loaded the ammunition. I’d be curious if they tracked down that lot and tested other rounds.

    • Chris Whiteowl

      Very thorough forensic breakdown. This was a treat. Not often do we get this kind of detail and knowledge in how a catastrophic failure happened. Thanks Phil.

  • looks like a over load of the ammo to mee i have seen a few of thos

    • sianmink

      any proper rifle powder, you can’t put enough in the case to cause dangerous overpressure.
      but 24 grains of pistol powder will turn an AR into a grenade.

  • noob

    It’s got the magwell fence so I’m guessing this is a post-86 lower and thus non-transferable.

    Still, considering that the weapon isn’t cheap, could this lower be salvaged and put back into service after an event like this?

    • TCBA_Joe

      Of course it’s not transferable. It’s owned by the Rio de Janeiro State Military Police. The weapon will probably be scrapped considering the damage and it’s age. Lot’s of smaller countries are finally buying newer weapons to replace their aging AKs and loaned M4s/M16s. Companies like Sig and Berretta are selling their new weapons to smaller countries traditionally armed with loaned or surplused weapons.

  • Ronaldo Olive

    For the record, the M16A2 Commando carbines were acquired new from Colt by Rio de Janeiro State some years to equip both the Civil and Military Police forces. I’m trying to get more info.

  • dp

    Perhaps one more possibility is there, before we consider Colt at fault which, as Cdn Phil points out, is highly unlikely. There is no chance for out of battery fire on AR15/M16 assuming the cam pin is in place and firing pin is not extra long. I was myself involved with in-depth tolerance analysis of the system at the same facility prior to Cdn production.

    The bolt is designed to withstand the FORCE obtained from cartridge and it does it well – see other derived designs worldwide. If however the GAS pressure directly enters upper’s area via say primer blow-up, is will just plain wreck it, regardless. In that case, the carrier will loose its guidance and wiggle itself loose. You will end up with what you see.

    • dp

      Even ruptured case close to base might be a possibility. If you look at shot of split receiver, there is evidence of heavy fouling close to chamber area. Check with Companhia de cartuchos.

  • Still a 10000 dollar receiver if it were transferable in the USA.

    • HSR47

      Your figure is massively low. Two years ago you were lucky if you could find a Sendra for 10k, an Colt SP1 for 13k, a Colt A1 for 16k, and a Colt A2 for 20k.

      These days you’re looking at ~16k for most non-colt A1 profile lowers, 15-17k for Colt SP1 guns, 19-22k for Colt A1 lowers, and 26k+ for Colt A2 lowers.

      • Tenacious221

        Right…but those are the “this reciever hasnt been blown up” prices…this one probably has function issues.

  • A Bore Obstruction, likely a squib round did that. Bad ammo.

  • I’ve worked on, diagnosed, and rebuilt a couple of blown up guns. It’s usually due to an over-pressure cartridge. The nice thing is, the way the AR platform is designed, you don’t tend to get many injuries when kabooms happen like this. Your cheek isn’t on the receiver, and your firing hand is down low below where it generally happens.

    One of the kabooms I rebuilt was a friend of mines. Shortly after the first Obama gun craze there was a LOT of bad Federal ammunition out there. A buddy if mine was taking a carbine course with some of it, and right in the middle of a firing string he blinks and he’s holding a gun that’s it pieces. The instructor yelled at him to go to his secondary so he slings the rifle, unholsters his pistol and continues the course.

    The instructor just thought he had a stoppage that he couldn’t clear and didn’t realize that the gun actually went kaboom.

    We replaced the bolt, carrier, and upper receiver and used a vice with padded jaws to squeeze the magwell back into spec, and he still shoots the same gun today 🙂

  • First Last

    Of course, if the cam pin in the carrier broke or was left out, the bolt could close without locking up. this would allow firing “out of battery” and would explain the damage seen.

    All we need is more pictures showing the state of the bolt, carrier, cam pin and firing pin…

  • Zapp Brannigan

    The bolt carrier is damaged on the right side yet the bolt lugs and barrel extension look intact. Usually in a case of an overcharge of gunpowder the bolt lugs get sheared off. Maybe there was some sort of existing damage or modification to the bolt carrier that allowed the bolt to move rearward in the bolt carrier but not rotate so it could fire without the bolt being secured to the barrel extension.

    The upper is destroyed yet the lower looks relatively undamaged. Also the cracking around the gas tube and barrel extension look different than the other portions of the upper that broke. Maybe there was a flaw in the upper near there or some other cause that weakened the upper.

  • Long

    It could be a squib that made it past the gas port opening but was still blocking the barrel. With nowhere for the excess pressure to go, it all came out through the gas tube into the receiver where it caused the damage we see here.

    • ThomasD

      Not all out through the gas tube. Just enough to drive back the carrier and rotate the bolt free. Then the remaining pressure came roaring back out the chamber and into the action.

      If you were to place a singe round loose in an open action, then cook it off, or otherwise smack the primer you’d never see this level of damage because the brass alone could not sustain the necessary pressure levels. It would be ugly, but also very dirty/sooty, and it might make the weapon non-functional, but it wouldn’t be such catastrophic destruction.

  • Ben

    looks like it needed a good cleaning first of all

  • Brad

    That’s what it looks like when you load and fire a .300AAC round into a 5.56mm chamber.

  • ThirtyCal

    I saw some reference to ‘squib’ rounds. I’ve been shooting, ‘a lot’ all of my life and have no idea what a squib is. Anyway, that is a heck of a lot of damage from 20 grains of propellant. My bet is they somehow got hold of some spiked rounds intended for bad guys. i.e. rounds that are filled with explosives and literally blow up with fired. Gov’s have been doing that to enemies for about 150 years. There are you tube vids where the jihadi’s mortar crews ‘blow themselves up’. Those were/are not accidental.

    • Pat Hines

      That, sadly, may be true. A squib round is one that is usually under charged with propellant, and when the next round was fired, you see the result. It’s possible for a squib round to cycle the action, but leave the projectile part way down the barrel, just beyond the gas port.

      • ThirtyCal

        Gotcha! Thanks for the intel re ‘squib’. I had an old .357 round go ‘pop’ rather than ‘bam!’ once. I unloaded the weapon and sure enough the projectile was hung up about 1/4 way down the barrel. I used a wooden dowel and WD-40 to drive it out. Fortunately the round was so weak it was very obvious.

        • Pat Hines

          Yes, I intentionally neglected to say, “been there, done that” in my post above. Don’t tell anyone.

      • no_tubes

        What happens with a squib, is that the operator gets a “click”, and does the tap, rack, bang drill. Only he didn’t bother to inspect the “dud” round and see that it no longer has the bullet, which is now lodged about 6″ down the bore of the rifle. When that next round is fired, kaboom.

        • Para

          Presumably, the kaboom comes because the expanding gas is now trapped in the barrel?
          What actually gives in the kaboom? I would have assumed that something gives somewhere and the full-pressure gas then goes into the receiver which is obviously not designed to withstand it. The sheer pressure forcibly unlocks and cycles the bolt?

          • no_tubes

            The gun in the pic was most likely OOB ignition.
            With a round fired with an obstructed barrel, especially with an obstruction close to the chamber, the barrel typically will burst.

  • Metuant

    Bad ammo, a case head separation, improper headspace. Observed this in M1 carbine’s as well.