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

[DeNinny]

Ninny,

The feed cone depth is headspace related. You can have correct longitudinal headspace and still have the case rupture because there is too much space around it - the case is too narrow or the chamber too wide. As shooters we more often refer to longitudinal headspace because it is easy to measure. And if horizontal headspace is OK then typically axial headspace is also.

There is still plenty of pressure in the barrel to do mischief as the carrier unlocks and the bullet is still in the barrel. Especially in a short gas system with a slow powder. Yes, the pressure drops as the bullet heads down the barrel but in this scenario it is still 39K lbs at the port, down to 16K lbs the muzzle (Ref: QL)

I can't disagree with any of this. Which is why I won't take headspace off the table. But again since feed cone depth is creating the largest area of unsupported brass, and since Bill Alexander had issue with this, I still think it's more important than headspace. And I still agree with mostly everything you stated.