Yep, a stuck case, just one, is worrisome enough.
So, you gonna remove the barrel, use the gauges, re-install the barrel? Like all belted magnums, it headspaces on the belt, right? You ever Mic the case just ahead of the belt to see if you are getting a ring there? I wiped quite a few 300WM brass with an out of adjustment die but it doesn;t take much to create that ring. And then you get a stuck case. Really stuck.