Originally Posted by F106 Fan
There are some cases, like S&B, where the primer pocket is very tight. The difference in force between seating a primer in a tight pocket and popping one against a SP case is not all that much.
One of many reasons you should sort your brass by headstamp and load one type of brass at a time. If it feels different, you know there's a problem.
The few times I've loaded mixed brass, it's been really disconcerting with some primers practically falling into the pockets and others needing to be seriously mashed in to fully seat.