Thread: What in the world?

My vote is that the case was sized too short and separated, then under inertia jammed forward into the shoulder.
I dont think pressure of gas outside the case could collapse it into dimples because it would be balanced by pressure inside.
The haze on the separated base suggests the body has been sized a long way back towards the belt.

Sizing too short has happened to me when the jaws of the case holder on the co-ax press dont open properly and the base hangs up too high so it come out about .200" short. Usually pretty obvious. Maybe you could get that if you used the wrong case holder like a 308 holder and the case went partway into it.
Or was this case actually fired in a 300 win mag chamber which is 0.160" longer ?

If the case was sized too short then measuring the two separated pieces should give < 2.500".

Anyway it's best to go to the reloader first. He might know straight away what it is.