Out of battery kabang.

[blade_68]

I think that Markm is spot on... A "hot" load many years back, I ordered 300 rnds of match load 308 that done many of the listed things in bolt smear of brass on bolthead, primer flow and blown primers then blown out extractor. That ended with a few hours pulling bullets and burning the powder. The loads was consistent on charges but likely to been mixed up on powder. On another batch of 308 had some hot loaded that blown out AR10B target's magazine with blown primers and casehead.

[ozarkpugs]

Was this a proven gun ? Someone suggested barrel out of specs is the reason I asked . you stated you found another case you think was from the same gun with missing primer so why would it not be possible for a primer or peace of one to ride with the bolt and cause an out of battery ?

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[HelloLarry]

Apologies in advance, I don't have pictures because they said no copies. I have not handled the parts and pieces.

Case head blew out, in front of case head there is about a 1/8 wide ring of expanded metal around the circumference of the case, to me this indicates how far the case was out of battery when it fired.

Shooter had shot 5 mags, of 8 available, before mishap. When policing brass one piece was found near the shooter's firing point with no primer in the primer pocket. Likely, but not certain it came from shooter's rifle.

Bolt carrier was cracked on the sides and bent slightly downward near the bottom. Remarkably, at least to me the bolt seemed to be intact. The firing pin also appeared to be normal and intact (no measurements provided). The extractor was bent significantly outward and the case head from that area of the case was missing. There was significant flow around the ejector which helped locate the case's orientation. Bore of weapon was clear.

I too believe this was an over pressure round.
The bolt started it's travel, but couldn't keep up with the speed of the pressure wave. This is similar to an OP round I had one time that took apart my rifle. This is a typical failure mode of ARs.

The only thing I see as possibly unusual is that the carrier cracked on the sides. Usually they split top-to-bottom from the cam pin cleaving them.

[MistWolf]

The kaboom would happen before the gas has a chance to reach the gas port.

[Todd.K]

When policing brass one piece was found near the shooter's firing point with no primer in the primer pocket.

Any ideas as to what could cause such an outcome?

Missing primers are a sign of excessive pressure. Assuming we are talking about factory ammo here.

You described exactly what I would put in a textbook on case failure due to excessive pressure.

[markm]

I'm wondering if there is any possibility that a primer somehow stuck to the breech face and set off the round being fed as the bolt tried to close.

If the round were ignited before the lugs locked, there'd be no kaboom. The round might burst, but it wouldn't damage the gun as described. Battery is required to achieve that much pressure.

[26 Inf]

Here's the case. How did that ridge happen if it was locked into battery?

Attachment 52018

[P2000]

There is a distance from the bolt face to the chamber wall where the case head and some of the body is not supported by the chamber. My guess is that this distance matches where the bulge is in the photo. Looks like the case was expanding in this unsupported area which would take a ton of pressure to occur, and maybe the brass could be partially to blame as well.

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[Todd.K]

Just over 1/8" of the case is sticking out of the barrel in the M16 design. A bit higher up the case and the wall thins significantly, this thinner wall is where we would expect to see the case fail if it was not fully chambered. Instead the case failed in the thickest part, but only after enough of the brass flowed out the only place it could. The extractor, right? That is where the case blew out? And there is some unusual brass coloration on the bolt and maybe carrier?

[AndyLate]

Every time I see a severely overpressure round like that, I am reminded how well the AR handles catastrophic failure, from a shooter standpoint.

Andy