Mk 248 Mod 1 is over typical pressures, and most likely will be loose on the primer pockets. It's high enough over pressure, that Black Hills, who designed the round to sling a 220 SMK at 2850, wont produce it commercially. So, you might make sure that the brass isnt toasted already.
I believe it is mk284 Mod 0 brass date is 2007/2008 on most. Primer pockets seem fine, a little snug. Not loose is any way.
Anyone know what the regular internal case volume is for a 300WM?
While I was waiting for my 190's to come in I was playing with 220's. getting stuck bolt with 70.0g H1000. I'm gonna back down a bit. But like I say, I was just messing around and it seemed odd.