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

[DeNinny]

Feed cone is completely separate operation from reaming the chamber.

Found a relevant thread on Sniper's Hide titled "Grendel-like Belted Magnum Problem" HERE.

I did not read the entire thread but based on the pics posted in it and this thread, I think this is a result of the feed cone area being unsupported and it could theoretically happen in any barrel (at high enough pressure). The belt as shown in the pics looks like it is in the same area as the feed cone. And it doesn't matter what the brass, there is going to be permanent deformation of the brass at that point provided the pressure to deform it is exceeded. In the Hornady case, the brass did not tear and instead formed that belt. In the Lapua case here, the brass literally ripped.