It may be a problem with headspace if the primers are backing or protruding from the primer pocket in the fired cases. It would take a heck of a lot of excess headspace for them to back out far enough to blacken the pockets and give a blast of gas to the hair. Are the primers pierced or have the primer pockets expanded or have the primers backed out of the pocket? Ammunition from the 50s that hasn't been stored properly may be the cause of the problem rather than excess headspace.

Jerry Liles