Thanks all for your replies. I'm gonna shoot it the rest of hunting season then take out to a local gun builder to see what they think and if I should set the barrel back.
It looks possible that those cases set back "in toto", that is to say, the whole case moved back to squash the primers, while encountering no "chamber wall cling".
What amount of headspace does a FL sized case exhibit? This would determine if your chamber is outsized, or the new cases undersized.