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

[LRRPF52]

I have been thinking that but with extraction happening well after the bullet has left the mouth, I see it as the extractor did the damage well after the actual kaboom.

To your point, though, the casing might have ballooned into the extractor itself and the sharp edge of it actually cut into the brass and that is where the initial blowout occurred. Then later as the case continued to tear, it tore at the feed cone score "line".

Can you elaborate on your own failure model here and see if our thoughts line up?

Extraction can’t physically transpire until the projectile has passed the gas port, otherwise there is no way for gas to flow back into the Stoner expansion system and cause carrier inertia.

No carrier inertia, no bolt movement other than thrust.

Extraction caused this case to fail. I know with absolute certainty the extractor ripped the case open because of the extractor shelf mark seen on the case head, clocked perfectly with the case shear.

This means the front of the case was happily stuck to the chamber wall still, under significant residual pressure.

The slower-burning AA2520 propellant hit the port, and the port is probably cut too large.

This allowed too much gas to flow into the expansion chamber too early in the cycle, causing the bolt to rotate and unlock, while the extractor held tightly to the case rim for dear life (O-ring installed, otherwise it would likely have just slipped off and caused a FTExtract).

The power of the excess gas was so significant, it drove carrier inertia so quickly that it had sufficient force to rip the rim off partially.

We wasted about 8 pages talking about brass strength and case webs basically, when the answer was right in front of us, had we been able to see the case head like that.