I had a partial case head separation using a 1.8oz buffer

[LRRPF52]

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Ok so I enlarged the view of the other failure and I want to point out a few more things that support the blowout (while still in battery) model.

Based on this picture, the blowout looks to have started where my red arrow is indicating. The brass is literally blown out and there is a flap section of it further away from the tip of the red arrow. This section was the weak point which again is right at the unsupported wall score line.

And the tearing of the brass continued along the score line from the unsupported wall. It ended approximately where my blue arrow is pointing.

Again, the blowout model is still at play here. IMHO.

Edit: Remember too, that the extractor part of the bolt has more room for the brass to expand than all the other areas where the lugs are. The extractor tip is literally smaller than a bolt lug, so the brass is going to expand there first due to the path of least resistance for it.

As the extractor pulled rearward with extreme force, the case wall thinned out and succumbed to the residual chamber pressure, even after primary extraction had occurred. The bolt is rotating almost immediately, so now shear forces are introduced as well, not just axial forces. Normally the rotational shearing force will contribute to a complete head separation and also break the extractor lip off of the extractor shaft.

This one all held together quite well. Normally if you have a case failure while totally in battery, the bolt gets welded or impinged to the extension by molten brass residue, and the gases will split the carrier in half.

Because primary extraction was in progress, it allowed the gases to vent out the receiver and down the magazine well, blowing out the magazine.

One of the most overlooked aspects of the AR-15 or any rotating bolt lock-up is the rotational force during an event like this, or even on guns that seem to run, but are excessively-gassed.

You can expect extractor failure with extractors getting beat up like that under tension from out-of-time actions.

It was a major problem for 14.5” M4A1s suppressed with the KAC can, as well as 10.5” guns-all of which are 5.56 NATO of course.

It led to the development of the LMT Enhanced Bolt Carrier Group, which elongated the cam helix in the carrier, and created a pre-vent gas relief in the gas path inside the carrier key/expansion chamber junction.

If you dropped that carrier into the OP’s gun, I doubt this would have ever happened.