Thread: What in the world?

Its a belted magnum which headspaces on the belt not the shoulder. This means any amount of over sizing the shoulders stretches the brass forward and thins the rear cup intersection where the brass is harder and more brittle.. resulting in that case head separation. Possible water or oil in the chamber could have also caused it as the brass didnt lock and seal to the chamber walls early in firing sequence the which those dents tend to point too..
If its new brass Id be checking it closely as it may be out of spec. If its reloaded brass then the resizing die needs backing out. If its old brass, check the rest of it for thinning lines or cracks above the cup.
Id also check the rifle out..check the head space is set up right and check for bolt lug wear and recession.