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

[DeNinny]

...
I’m suspecting the feeding cone not supporting the case. Brass is inherently soft, so it will push around even with low pressures. When unsupported, it will yield like this.

This is exactly where my head is at with it too. And it was possibly exacerbated by a relatively "hot" load. And possibly exacerbated by brass with a relatively thin wall at the failure point. And it was possibly exacerbated by heat.

And all of this is anecdotal. The only way to know for sure is to repeat the experiment with changing conditions, one at a time. For example same load, same barrel, but different case. Or same case, same barrel, but different load.