Mystery Boom: Guess The Failure

    In a post on, 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



    Guess The Failure @TFB – Credit: IronBalaclava



    Guess The Failure @TFB – Credit: IronBalaclava



    Guess The Failure @TFB – Credit: IronBalaclava



    Guess The Failure @TFB – Credit: IronBalaclava



    Guess The Failure @TFB – Credit: IronBalaclava



    Credit: IronBalaclava



    Credit: IronBalaclava

    Compressed extractor spring:


    Credit: IronBalaclava



    Credit: IronBalaclava



    Credit: IronBalaclava



    Credit: IronBalaclava

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


    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.


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