CJF,
You might recall I asked about the rim thickness on your ammo, because I found that mine accepted 38-55 rima whereas I had to thin the same rims for 6.5x48R, 6.5x58R,
8x57R/360 and 8.25x57R Hagen. If your loads have enough of a shoulder that it is set back in sizing and the cases have thin rims, causing case stretch, your initial thought may be the cause. I suggest you smoke the neck (or ink it) of a fired case and size it just to where it "kisses" the neck and compare it to one sized as before. A pin prick hole would be more consistent with a partial head separation than excessively high pressure. You can also make a chamber cast to discover the true chamber length. It is not unusual for new cases to need trimming to length. I hope this helps, but if the cases had been fired many times the loss of one with no other damage is not especially alarming.
Mike
