Differences in primer pocket dimensions. Military brass is typically crimped, and the crimp can vary, even between years of LC brass. Then you have some brass that is once fired out of beltfeds, which can have pretty bloated case bodies, adding to even more variance in feel on a manual press (which is where rollsizing can be really helpful). Especially the lighter presses like the 650/750 and 550.What issues will I run into with mixed brass that I should be aware of ahead of time