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

[stonehog]

Another cause of unexpected high pressures is failure to trim the cases.

When the case mouth contacts the start of the freebore, there is no space for the neck to expand and release the bullet.

One can see much more, even double, the pressure increase we get with no jump when the mouth is pinned.

This is an interesting point. I just went back and checked the case length on all the rounds in my impacted box. the 3 that were showing signs of case stretch all came in over 1.520 max case length! The other cases were all under 1.5175. This may be an additional point in the stacking conversation...

When I prep new brass, I tend to take samples on the first X items - usually 10 or more, and then just do random samples after that. Looks like I can add another "new case prep" item on the checklist to make sure all cases are within spec for length. Honestly - it seems like most brass these days is coming in around .010 under max case length. I also don't know if these cases "grew" on firing to there extended length, but this is easy to measure on the other 150 loaded rounds.