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

[lazyengineer]

At those pressure levels, far more damage would have happened.

There had to be some pressure relieved, but enough residual pressure to allow gas escaping from the case to participate in the partial case head separation and flow around the bolt.

Earlier case failure isn’t supported by the way everything is presenting, the intact extractor being the biggest piece of evidence.

If you have seen in-battery/over-pressure catastrophic failures, you would see the distinction.

I don't think this was an over-pressure catastrophic failure. I think it was an in-pressure brass failure. Extractor survived because bolt was locked and it couldn't move much, and the relatively lower pressure than most kaboom's vented into the star-chamber and blew out between the opening between the lug-clearance passage, and then down the mag-well. Notice how Lapua Kabooms don't result in much rifle damage; where'as most AR Kaboom's are due to massive overpressure events like barrel obstruction or powder error, and tend to grenade, split the BCG, etc. Lapua Kaboom's, tend to not, because they're in-pressure case-failures that vent the lower pressure, and split much lower in the case body in a manner inconsistent with most other kabooms, because they can fail while still locked (in some BBL's).