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

[DeNinny]

Extraction contributed to the failure.

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?