7.62 AR-10 mild Kaboom

3 weeks ago we had a case burst in one of Pappabear's LMT MWS AR-10s.

Curious to get some thought on this since it appears that the case burst out of battery. At first I thought the case neck separated, but on further review, the case fire formed about 1/4" out of battery which makes the brass appear to have no neck.

The barrel and lower appear to be ok, but the upper receiver and mag were effed up pretty good. LMT took the upper back to inspect.

The round that burst was the tail end of an old lot of 168 SMKs, again, loaded with a chargemaster. And no other rounds had any signs of pressure.

Firing out of battery is nearly impossible, but will a kaboom allow the bolt to unlock before the case fails? A friend thought possibly a half/light charge could have been the issue.

https://i.imgur.com/ADC3XhM.jpg

Brass was MEN which had already been fired at least once with the factory loading.

Barrel obstruction does NOT appear to be the issue. We were slow firing long range and didn't have a missed impact on the previous round, and the suppressor suffered no damage.

https://i.imgur.com/qFSPXzp.jpg

You can see at the rear of the case how much of the brass was unsupported by the chamber when the case burst. The explosion happened when I was on the gun and it was on a clean trigger break... i.e. not a slam fire or something like that....

No blown primer or swipe on the head? Never had a kaboom before, so I'm not sure if that tells me anything.

Broken firing pin?

We were unable to get the BCG out to inspect. Oh yeah. No apparent bolt damages from what we could see. The extractor was not blown out like a lot of kabooms in battery.

Wow, that's some tore up shell casing!

16-inch barrel with a suppressor, standard buffer weight?

Yes, yes, and not sure on the buffer. Might be a heavy buffers deal. In fact, I'm pretty sure that it's the heavy buffers solid steel thing.

So, I just read the OP for the fourth time and realized you pulled the trigger when the kaboom happened. Which would mean that it was completely in battery, right? From looking at it, it looks like an over pressure occurred.

So that really only leaves a few possibilities, no? Some sort of barrel obstruction seems unlikely, given that your previous bullet struck the target, leaving the possibilities of a bad neck crimp and bullet setback or improper powder.

Is it possible that some pistol powder could have gotten in there?

ETA: Looks like you can get a secondary pressure peak if using a slow powder and an under filled case.

The LMT factory gas tube for their 16-inch looks to be the same length as an Armalite mid-length (to fit under the rail).

Add the back-pressure of a good/efficient suppressor and a standard-weight buffer and you're going to get accelerated bolt carrier movement to the rear and really violent primary extraction while the case is obturated against the chamber walls. Powder that would be burned up in an 18, 20, 22, or 24-inch bore (with rifle-length gas tube) is going to still be burning inside your suppressor, adding to gas volume in the gas tube, key, and carrier before the bullet uncorks.

An XH (Extra Heavy) buffer will delay unlocking somewhat, but the shorter gas tube is still going to be pumping expanding gas towards your bolt carrier group.

Let's look at your case. Primary extraction is partially yanking that case to the rear, pulling it out to where thickest part of the case head and body clears the chamber mouth chamfer and the case blows circumferentially where it thins. The neck lets go at the shoulder-neck juncture (and may stick in the chamber) or it blows out to to fire-form to follow the shoulder angle.

The extractor isn't blown out because the case head separated and the primer held in the pocket.

I don't think LMT sells their suppressor matched to their 16-inch barrel (in fact, I don't see their suppressors listed at all on their site any more). KAC seems to have fixed this with their 16-inch rifles by using a rifle-length gas tube.

What powder did you use?

NOTE: I AM NOT AN ENGINEER -- just a guy with guns and bullets.

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Originally Posted by georgeib

Which would mean that it was completely in battery, right?

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I believe so.

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ETA: Looks like you can get a secondary pressure peak if using a slow powder and an under filled case.

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One of our buddies is floating that theory.

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Originally Posted by sinister

The LMT factory gas tube for their 16-inch looks to be the same length as an Armalite mid-length (to fit under the rail).

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Oh yeah... BRT reduced gas port gas tube installed.

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Add the back-pressure of a good/efficient suppressor and a standard-weight buffer and you're going to get accelerated bolt carrier movement to the rear and really violent primary extraction while the case is obturated against the chamber walls. Powder that would be burned up in an 18, 20, 22, or 24-inch bore (with rifle-length gas tube) is going to still be burning inside your suppressor, adding to gas volume in the gas tube, key, and carrier before the bullet uncorks.

An XH (Extra Heavy) buffer will delay unlocking somewhat, but the shorter gas tube is still going to be pumping expanding gas towards your bolt carrier group.

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That Heavybuffers thing does give a lot of bolt bounce, but that seemed to go away with the BRT gas tube. But your notion would explain why it burst in the ejection cycle.

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The neck lets go at the shoulder-neck juncture (and may stick in the chamber) or it blows out to to fire-form to follow the shoulder angle.

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I was sure the neck was stuck in the chamber at first. The case looks short due to the peeling back by the head. But it is indeed actually fire formed out.

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The extractor isn't blown out because the case head and primer held.

I don't think LMT sells their suppressor matched to their 16-inch barrel (in fact, I don't see their suppressors listed at all). KAC seems to have fixed this with their 16-inch rifles by using a rifle-length gas tube.

What powder did you use?

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powder is Varget.