I have some military cases. I got the old primer out with some difficulty and I can’t get new primers to seat. Do military cases use different size primers than commercial brass?

Military cases have crimped in primers. The crimp has to be removed before primers can be seated. The crimp can either be cut out or swaged. Several companies offer a variety of ways to remove crimps. They range from a hand held tool, which actually cuts the crimp out, to primer pocket swagers that mount on your press. The swagers actually press the crimp out smooth.