I have the same problem ongoing now with one of my Son's 7x57's.
He's going to bring the rifle and ammo with him when he comes up from Detroit next month.
It is a headspace problem.
The load is 48.0 gns of H-414, with 140 gn Nosler Partitions.
I full length sized the brass, and I think I bumped the shoulders back too far. I load for a number of 7x57's, and wasn't paying attention to his rifle.
