RE: Blew a primer!!!!! I'm such an idiot. Update on page 2.
I would pull the bullets on the rest of that batch and weigh the charges again. The fact that the cases were not new should not cause that problem-just the opposite, in fact. Because the once-fired brass has a bit more capacity than a new case.......
Since the 300 Wea. Mag. headspaces on the belt, the fact that you might have set the shoulder back a little will not cause blown primers. Head separations, maybe later on.