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

[LRRPF52]

Sorry for resolution. You can look at OP's first post of the picture and you'll see the same thing there, without arrows.

I'm sorry but the flap could easily occur from the blowout itself, when the pressure is much higher than during extraction. Because of this, I think the blowout has a higher likelihood of creating that flap instead of the extractor pulling on it.

It is 100% impossible for that flap to be present while in-battery, even in a sloppy, out-of-spec chamber. Again, if you had a case failure in battery, where do the back-end gases go?

At those pressure levels, they will destroy the extractor, usually split the carrier, bow-out the upper, bow-out the lower, send fragments from the carrier and bolt in different directions, with magazine blow-out being the least of your worries.

This event was after that possibility, with enough carrier inertia to have cleared that much of the case head so the brass could fold back from the initial failure point.

Has nothing to do with headspace or case web support by that point because it’s partially extracted.

Lapua brass likely saved his firearm from further damage.