Blown primer was worse than I thought

[QuangTri]

Rifle is my frankenstein AR-308 that started life as a DPMS LR-308 circa 2007. Been through several barrels and stock configurations. Still using the original receivers and bolt group.

Anyway, last range trip or two noticed a bit of cratering around the primer. Less so on surplus M-80 than commercial. No issues other than that. Rifle is MOA with the current Fulton Armory Criterion 18.5" barrel.

Last range day, had a jam and noticed smoke coming from the upper-lower receiver. Cleared the jam and continued without incident. Later noticed the blown primer and even later that the case was split.
Attached photo of that and another case from that lot of Privi Partisan 165gr ammo.

What would cause such a thing? Never checked headspace, have some gauges on the way and a JP high pressure bolt to be installed.

[Suck My Glock]

My first suspicion would be that the chamber mouth is too generously belled. This is often true of many automatics designed for harsh use like belt-feds. In such designs, accuracy is not as important as reliability and so the chamber is ever so slightly larger than need be in case the round gets chambered with some crud or grit. This is the entire reason "small base" dies exist, so that brass fired in such over-sized chambers can be reduced down to standard dimension to work in more standardly tight chambers.

I have owned a few M1 Garands with generous chambers like this, but other than being a pain in the butt to resize, I've never experienced a case split like that. Perhaps it is a combination of the generous chamber AND hot ammo? Either one by themselves might not cause a problem, but together,....

[Doc]

Was this ammo purchased during the last 3 years?

I’ve been reading quite a lot about QC issues

[QuangTri]

Had the ammo for several years, had to wait until I was in Arizona to do some good shooting.

The barrel is a quality part, made by Criterion for Fulton Armory. Not a military part.
Have had no issues with it in the limited firing before moving it here. Sporadic accuracy was cured by bedding it to the loose DPMS upper. Lots of different brand .308 and 7.62mm through it without event.

[Old Jeff H]

Man, I don't know. I've seen neck splits and case-head separation, but that vertical split has me scratching my head.

Did a quick search to see if there were any PPU recalls, but not finding any.

I don't see any flattening of the primer?

I guess to me it looks like the case head isn't being properly supported?

[QuangTri]

No other brand of .308/7.62 show any signs of incipient failure, and I do pick up my brass to check it out. The possible primer cratering on commercial ammo was all I noticed, and surplus M80 does not show that.

[xerts1191]

I vote ammo SNAFU

[omegaman]

Shoot long enough it will happen.

[QuangTri]

Got my new JP high pressure bolt in and now not a sign of primer cratering. Shot up the rest of the PP ammo without incident.
I do think I will avoid buying more of it and stick to Winchester white box for target shooting.

[condition_0ne]

Is the PPU loaded to .308 specs or 7.62x51 specs? M80 is loaded to 7.62x51 NATO specs and as such is slightly milder than civilian .308 Win. chamber pressures. Usually no issue, but might be an edge case in this instance. What bullet weights for the two different loads? That split case head and the specific location of the split looks like the bolt might be unlocking a little early and the case head would be unsupported as it unlocks.

[QuangTri]

Am back in Hawaii just now, and do not have the PP box.
If memory serves, it was labeled 7.62, though with 165gr boatail. so not equivalent to M80.
Since nothing like that has happened in a few hundred rounds, only light primer strikes(my fault trying to use a too light hammer spring) and the mentioned primer blanking/cratering.

The remaining PP ammo shot fine with no primer indent weirdness, so I am thinking it was just a faulty case. I run an adjustable gas block so premature bolt unlocking would not seem to be possible or I would have seen some signs by now.