The "deep primer strikes" got me thinking that maybe someone had put in a 7.62x39 firing pin instead of a milspec firing pin. Hoever on an AR the firing pin protrusion can be changed by shaving the rear of the bolt (to increase protrusion).
Normally a deep pin strike would cause a failure to the impact site, not necessarily the sides of the primer. But I guess some blow by is to be expected, I've got a lot of old Mauser's with a perfect ring of scoring around the firing pin from blow by.
Machine guns are awesome until you have to carry one.