Mystery Boom: Guess The Failure

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In a post on AR15.com, a gunsmith going by the screen name “IronBalaclava” recounts being presented with the aftermath of a catastrophic failure. The customer was shooting three rifles that day: two chambered in 5.56 and one in .300BLK. The incident occurred while shooting the 5.56×45 AR15. The owner of The ARmorer’s Wrench LLC in Ann Arbor, Michigan writes:

Customer on the range comes out with his brand new Sig 516 5.56 with the bolt stuck about a 1/2″ partially out of battery. I go in the range and mortar the rifle about 8 times HARD to get the bolt to come back. It finally comes back and this is what I see(he had 3 ARs out and yes, one was chambered in .300 Blk, so you can guess what happened):

The casing is stuck under the extractor and attached to the bolt face, so I pry it out with my Leatherman and take an initial pic of the case:

I asked what happened, and he said he squeezed the trigger, heard a “pop”(like a squib load)and saw lots of smoke come out of ejection port. No mention of the mag blowing out. I inspected the bore/chamber, and it was clear. No spectacular Kaboom with a peeled back receiver and trashed BCG. No elongated, stretched out. .300 BLK round lodged in the barrel. Where did the projo go?

After field stripping the rifle, everything appeared visually sound, except the ejector spring was trashed and lost all tension. Visually, the extractor and bolt appeared unharmed. I logged the rifle in my ‘smithing Bound Book and took the rifle in to the shop and the following is what I observed.

A couple pics of the casing. Looks like a cut/resized PMC .223 case. The primer was blown clear of  course, but no remains were found anywhere:

I checked headspace, and it passed the Go, No-Go and Max gauge. Replaced the ejector, ej. spring and ej. roll pin and it’s hand cycling just fine now. What’s with the “pop” and no projo lodged in the barrel? If it actually went bang and not pop (I wasn’t there to witness the event), Why didn’t the rifle blow the **** up?

Before you scroll through the pictures, take a guess at what happened. The headstamp is .223 and the barrel is chambered in 5.56. No bullet was lodged in the bore.


Signs of overpresssure:

Guess The Failure @ TFB

Guess The Failure @TFB – Credit: IronBalaclava

 

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Guess The Failure @TFB – Credit: IronBalaclava

 

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Guess The Failure @TFB – Credit: IronBalaclava

 

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Guess The Failure @TFB – Credit: IronBalaclava

 

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Guess The Failure @TFB – Credit: IronBalaclava

 

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Guess The Failure @TFB – Credit: IronBalaclava

 

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Credit: IronBalaclava

 

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Credit: IronBalaclava

Compressed extractor spring:

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Credit: IronBalaclava

 

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Credit: IronBalaclava

 

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Credit: IronBalaclava

 

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Credit: IronBalaclava

Ok, after reviewing the images, did you change your mind on what happened? How about this comparison picture:

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Credit: IronBalaclava

Our damaged case is on the right, a 5.56 case is on the left – clearly a 300BLK in a 5.56 gun. But where’s the bullet?

To be blunt, The shooter pulling the trigger failed.

Be careful everyone.



Pete

LE – Science – OSINT.
On a mission to make all of my guns as quiet as possible.
Pete.M@staff.thefirearmblog.com


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  • Disarmed in CA

    Gonna have to start putting training wheels on those things

  • Justin

    I don’t get it… where is the bullet?

    • Xtorin O’hern

      somewhere down range

      • Blake

        There’s no way. If it was indeed a 300 BLK round it would be physically impossible for it to fire all the way out of a 5.56 barrel.

        • Risky

          With enough pressure before a part of the gun fails, the force can swage a larger diameter bullet down a significantly smaller bore. Can easily happen when firing .308 Winchester out of a .270 Winchester, for example. I think when this happens with .300 Blk in a 5.56mm, the result usually is that the swaged and elongated .30 caliber bullet just gets stuck in the bore. In this case it might have had enough pressure to completely exit the bore.

          • Blake

            I highly doubt it actually went all the way down the barrel. There’s so much less powder in a 300 BLK round than even a 5.56 round there’s no way it had enough power to squish the round and get it all the way through. Especially since there wasn’t any damage to the bore or rifling.

          • JoelM

            I have seen .45ACP fired in a .308 Winchester where the bullet swaged down to .308″ and left the barrel and put a 3 inch long hole through the 25yd target board. I don’t think I have to tell you how little powder a .45ACP uses compared to a .300 Blackout.

  • Jeremy

    I am still stuck on where is the bullet too. The 300 BLK clearly resized to the 5.56 chamber. But I have trouble imagining that the bullet did not get jammed in the barrel. Was it a subsonic 300blk? I tend to think even a sub would clog the barrel

    • Xtorin O’hern

      it was a 5.56 in a .300 chamber, the bullet is somewhere down range

      • GI

        Look at the picture of the barrel. It is marked 5.56.

        • Xtorin O’hern

          did you even read the end of the article? they even tell you what happened

          • GI

            Did you? Best read it again genius. They said a 300 in a 5.56,and you said a 5.56 in a 300 even though the barrel says 5.56…

  • Jim B

    No blood = good day!

    • Phillip Cooper

      Yep. Go buy a lottery ticket, you’re having a lucky one.

      • Gary Kirk

        Yep, just lost at least a few hundred.. Might as well take a shot at getting it back..

  • Joe Liberty

    The bullet got pushed back into the case? Combined with an OOB firing b/c the case wouldn’t chamber, obvs. Enough pressure to blow out the mag but not enough to blow up the gun, since there was no projo in barrel. Based on headstamp, it’s a reload, so maybe the shooter cut the case to length and never resized so no neck tension?

    • Rick O’Shay

      If they never resized the neck I’m not sure how that’d work. The moment you tipped the bullet sideways everything would fall apart.

    • Phillip Cooper

      He never said the mag blew out.

      • Joe Liberty

        Hmmm. Still think there’s a bullet in that case tho. And maybe it was resized, but for some reason there was not enough neck tension. Wrong projectile or maybe seated way too deep?

  • Joseph Goins

    Stupid is as stupid does.

  • Vhyrus

    Did he put a .223 bullet in a 300 BO case possibly? That would explain no kaboom and the reformed case. The shorter case wasn’t seated properly and formed to the chamber when it fired.

    • Anonymoose

      Actually, that gives me an idea for a wildcat cartridge. Cut a 5.56 down and we could give the Commando/Mk18 the ballistics it deserves without the fireball that entails with full-power 5.56×45. Since the 5.7×28 goes subsonic when you load it with 55gr boolits, if I could create a 5.56×35 round that pushes a 62gr at 2600fps out of a 10 inch barrel with less concussion and flash, there’d be no point for PDWs anymore (if there ever was).

      • iksnilol

        You mean 5.56 Minsas?

        • Anonymoose

          Ah, crap, I forgot about that one. I still think we could do it better if we used heavier bullets, and the MINSAS only gets 2300fps with M193 bullets.

          • iksnilol

            You also have .221 Remington Fireball. 5.56x35mm that one.

          • Anonymoose

            Looks like everything has already been done then, except for loading heavier bullets and using a higher twist rate. 😀

          • iksnilol

            Basically. I am getting depressed now. Only thing I am good at has plateaued and everythang has been done.

          • Anonymoose

            Welcome to the club.

          • iksnilol

            *amount of sljivovica in my evening tea increases*

            Yeah.

  • Rick O’Shay

    If I had to guess? Maybe it wasn’t a 300blk round at all, but a .223 that experienced case separation? With the top half of the annealed case still stuck in there somewhere, somehow? Or pulled/extracted out on the next round? A 300blk bullet doesn’t just magically vanish being shot down a 5.56 barrel, certainly not with a “pop” like a squib. There’s info that isn’t being shared, either by the shooter or the person he shared this stuff with.

    • Jeremy

      You can see where the case used to have the 300BLK shoulder and it was fire formed out. It was def a 300BLK brass at one point in it’s life.

      • Rick O’Shay

        Then I’m at a loss. I’ve seen some annealed cases that, when cut for 300blk, have the old annealing line up right near the shoulder. If that was the case here, I could see it giving the appearance of form-fired 300blk brass. That’s the only stretch of the imagination that makes sense to me. The shooter this happened to, is leaving out some critical info to save face on his stupidity otherwise.

    • Pete – TFB Writer

      That was my first thought too.

  • Jean Welter

    bullet set back all the way into the case. Pop and smoke were blowby?

    • Risky

      But if that happened you wouldn’t have any of the signs of overpressure.

  • spotr

    Perhaps… A shooter that has 300 blk and 5.56 .. may have accidentally “reloaded” a case sized for 300 blk with a 5.56 projectile. Long shot since the reloader would have (or should have) noticed the large effort required when reloading. The end result would be a shorter 5.56 cartridge. If this was the case then the projectile went down the barrel. It was 5.56. The case “blast formed” to the straight sided end result since it was shorter.

    • Trey Heldmann

      This gets my vote as well

    • Repoman3737

      Wouldn’t a 5.56 bullet fall into a case sized for 300 blk with no effort since 300 is larger than 5.56? Now a 300 bullet into a 5.56 case would take enormous if not impossible effort to seat. I think it was a soft round or a hollow point that squeezed down to 5.56 from a 300 or it may have broken up as it squeezed.

    • JoelM

      It’s physically impossible to cut a .223 casing off before the shoulder (as is done to convert to blackout) and then reform a new shoulder in a .223 die. There’s no possible way to push the case deep enough into a resizing die to do that.

      • spotr

        I was suggesting a 300 blk case (already formed to 30 cal), rather than a straight sided home made 223 cut-off. The question would be – can you put a (preformed) 300 blk case into a 223 re-loader and have the resizing die squeeze down the last .084 of dimension change?

  • Vhyrus

    Is it me or is there something strange about that bolt carrier? The gas key looks… not all there.

    • Tony

      The Sig 556 is piston driven, no gas key, just something for an op rod to smack against.

      • Tony

        516* my apologies. Statement above applies to the 516 model.

  • Pete – TFB Writer

    Bullet is not in the case. Good thoughts though.

    How about a frangible round?

    • Rick O’Shay

      Made of what, chalk???

      • Pete – TFB Writer

        Crackers.

        • Gary Kirk

          No cheese?

          • Nashvone

            Peanut butter.

          • Gary Kirk

            Sorry.. I’m all high class and what not.. I normally take my guns to the melting pot.. No peanut butter

          • Pete – TFB Writer

            Fine. Spreadable cheese.

          • Gary Kirk

            Fine. SPREEEEAAADABLE cheese..
            If ever there were a need for excessive CAPS on this blog.. LOL!!

    • DrewN

      Or a sabot maybe? But yeah, that is what I was thinking.

      • Rick O’Shay

        Holy sh** that would take some stupidity… loading a .223 sabot into a 300blk case, and then deciding that was good enough for shooting out a 5.56 barrel.
        Like, a level of stupidity beyond not keeping track of your mags and ammo when bringing multiple caliber ARs to the range.

        • Gary Kirk

          https://uploads.disquscdn.com/images/a0a361ceb0a2aca911ff7e6e0f48fc19183aa0da00c78b85b9e0f749538270c4.jpg

          What if they pulled some of these, then figured out a way to reload them

          • Devil_Doc

            Take a lightweight high velocity 5.56, convert it to fire .30 cal to take advantage of heavier rounds, then load it with lightweight bullets designed to be fired at incredibly high velocity?

          • Xtorin O’hern

            technically if i’m remembering my science right the sabot could theoretically go faster than a normal 5.56

          • Gary Kirk

            The sabot in the accelerator rounds I showed use a .224 diameter core in a .308 bore diameter sabot.. Not really sure about all of it, just remember the ammunition.

            Pretty much a 55gr 223 round fired from a 30-06.. Don’t think they ever worked right because of twist rates in the parent cartridge barrels.. But could be an interesting development for 300blk

          • Komrad

            there are places you can buy newly produced sabots with load data
            just not completed ammo

          • Gary Kirk

            Interesting.. I might be into a 300 blk if that’s the case.. Same loading for both supersonic and subsonic loads perhaps?? Need more info on those sabotage bullets please

          • Gary Kirk

            Again, if you could, please give me some details on where. I am actually very interested

  • He better change the bolt out, and barrel extension if he is smart. They may head space correctly but that does not mean they are not filled with micro cracks now. Mag Particle test would tell but its would be cheaper just to trash the bolt and put a new one in. Might have been a subsonic round and when they mortared the action the bullet went sailing w/o their notice.

    • Gregory

      I agree 100 percent, better safe than missing body parts.

    • marine6680

      I expressed that concern on the thread myself.

    • RSG

      Lol. Said the same as I was looking at the pictures of the bcg

    • Gary Kirk

      Now it’s truly MPI.. Major Problem Incorporated..

  • Rob

    Possibly the bullet was completely resized and made it out of the barrel at a very low velocity. That would explain the strange sound of the report.

    • Sianmink

      That would still be a loud bang, not a pop. The bullet had to exit fairly rapidly, or the magazine would have blown out.

      • Rob

        The bolt and cartridge case didn’t fail and the extractor was still attached and in one piece. The primer backed out, but it doesn’t appear that there was a lot of high pressure gas loose in the action. The primer probably fell out when he mortared the rifle to open the action. The primer loosened due to case head expansion, but I think it stayed put on the bolt face until they opened the action. My guess anyway.

  • Joe Liberty

    Sig 516 comes in an SBR model. Maybe the barrel was short enough to let the bullet out and drop pressure before the gun blew up?

  • Graham2

    A soft lead .30 calibre bullet in the 300 BLK case, soft enough to squeeze down the 5.56mm barrel.

    • HSR47

      This would be my bet: a light .30 Mauser/Tokarev or .32 ACP bullet in the ~78-92 grain range resized down to .308.

      The 5.56 NATO barrel might be enough to resize such a bullet down to the proper diameter to make it out, especially with softer lead.

  • Raoul O’Shaugnessy

    Had a buddy drop a .308 Norma into a 7mm STW. The bullet actually exited the barrel and POI was only of a couple inches from the rest of the group. Of course, he had to hammer the bolt open. .308 to .284 isnt nearly as a big a jump as .308 to .22, though.

  • Dan

    It either squeezed the 300 slug down the barrel or it was swallowed in the case and then fell out in the excitement following the baby-kaboom.

    One possibility if it squeezed it down the barrel follows: Maybe the DI kabooms we’ve been seeing happen when the following occurs: 1) chambered 300 blk fires and starts to try to push down the barrel. 2) Strong chamber and bolt keep it all buttoned up as pressure soars. 3) As the 300 slug starts to form and push down the barrel, the overpressure in the case builds up, blows out the primer, and then exits into the bolt through the firing pin hole. 4) This pressure is enough to make its way into the BCG and operate the bolt-opening process as though the BCG was getting pressure from the barrel/port/gas tube. (and BTW, this jet of hot gas would probably anneal the springs in the bolt). 5) This causes the bolt to open while there is massive pressure and the 300 slug is trying, relatively slowly, to make its way down the barrel and equalize the pressure behind it and in front of it. 6) The bolt opens enough while there is lots of pressure still there to blow out the case, magazine, upper, eyewear, etc. BUT…. and it’s a big but…. if it’s a piston gun, there aren’t piston rings etc. in the BCG to allow the primer overpressure to open the bolt. It all stays buttoned up until the newly-minted 5.56 round gets down the barrel. By the time it passes the gas port, it may or may not have enough pressure to push the piston hard enough the eject the heavily expanded case. One bit of evidence for my hypothesis, though not definitive, would be if we have seen 300blk/5.56 kabooms in DI rifles only, or if they have also occurred in piston guns. Anyone want to donate a 516 or an MR556 to test???

    • Gary Kirk

      Good point

    • Xtorin O’hern

      it was 300 in a 5.56 chamber

      • Dan

        Yes. This whole explanation involves a 300 round in a 5.56 gun. Google around. There are cases of a 300 round getting forced successfully down a 5.56 barrel in a kaboom.

  • Gary Kirk

    If there was damage to the mag.. Maybe the bullet is in there?? Undersized cartridge, goes semi to battery, fires, never fully enters barrel, almost a blowback kinda deal, bullet falls back with case, ends up in mag?? EXTREME STRETCH HERE!!!

  • Frank Grimes

    Between the stupid gimmick caliber choice and stupid gimmick external piston AR15 clone, I’m surprised the obviously special needs owner was allowed to leave his assisted living facility to go to the range.

  • gunsandrockets

    Now that I think about it, the same kind of failure with a .300 fired in a .223 could happen with a .223 fired in a .204 Ruger.

  • Tony O

    Well, this is all enough to convince me I don’t need to add another caliber to my collection. Sticking with 5.56!

  • Mystick

    Having the receiver stamped “.223” and the upper being something completely different?

  • Stephen Paraski

    get a real .308.

  • Dee Carlile

    Would a 125 gr, .30cal bullet, which would have a varmint thickness jacket, survive being resized to .22 cal and exit the barrel? That’s my guess. Keep your 300 guns and ammo away from your .223.

  • Jake

    Is it possible the round wasn’t completely chambered? Hence the rifle wasn’t in full battery but maybe just enough to strike the primer, sending the bullet down range and re-sizing the case to the chamber?