Sounds to me like you need to trim and full length resize the brass, if it's too long either in overall length or to the shoulder, the action won't lock up completely and it'll pop open every time, being too long means it's likely been reloaded too many times, start with new brass and make sure there isn't excess headspace, chambered brass should be flush with the barrel face, not recessed much if any, nor above the chamber face.
Tim