Tom, True nuff. I've heard of that as well. I've only tried the one brand of ammo in this gun as yet (only had it a short time). Fired one round then cleaned the barrel after each shot three times in a row. All the primers were pushed back around the firing pin in the little gap between the pin and the hole. The primer itsself was still in place (didnt push back out of the case). No unusually large gap between the pin and hole. But this particular gun has a REALLY long firing pin. Sticks out of the bolt further than any gun I've ever seen.
It's a 91/30 built late in the war in the Izhevsk arsenal from new parts from both there and the Tula arsenal with a sniper grade barrel (has the 0 marking on it), and a 1928 stock. EVERY part is from a different time and place, but all numbers are original stampings and match! They never issued it and the rifling is as sharp as a razor, but the bore is as pitted as a gravel road. Funny because NOTHING else show's signs of rust. Just the bore. Test fired with corrosive primers before storage I guess.
Could be low pressure, but with modern factory ammo (soft primers), and the extra long firing pin I'd bet its stretching the metal of the primer. Add the rough bore, and you get a scary effect! The three shots it fired were about three quarter inch at 50 yards (iron sights, so probably mostly my fault). I may shorten up the firing pin a bit after trying other brands of ammo. Even the individual parts of the bolt are from different times and arsenals! All look unused though, and the action is smooth as butter. Weird.
With that FiveseveN, a 1/8" per firing stretch is WILD! Are you measuring them after firing, or after sizing? Are you guys doing any hunting with this caliber???