had that happen with some 5.56 I had loaded by a "trusted" person.yes, yes, I violated the cardinal rule of " if you didn't reload it yourself, don't shoot it" fortunately both me and the rifle survived, but it shook the hell out of me. Follow survivalshop's suggestion, and see if they seat properly and evenly. If not, there is either uneven material under the primer, or uneven pressure being exerted to seat it.