OzFclass

I used to run Berger 88 varmints in a 6br. It was long with a relatively flat base. The seating depth I arrived at meant the base was sitting right at the neck shoulder junction. When measuring seating depth I found some bullets would pop back up by .006" or so.
I determined this to be due to a slight radius at the neck shoulder junction and the bearing surface was just starting to be seated into this radius.
Moral of the story for me - stay away from the neck shoulder junction

Aligned Tim