Since the 7.62x54r round is rimmed a thicker rim would cause insufficient headspace. A thinner rim would, on the other hand, could cause excessive headspace. I've shot some of this ammo in the past, and had quite a few problems with it. It is just old, bad ammo that should be junked.
This is true too. The Russians were building these gun quickly, and were not all that picky about QA standards during war time production demands. I had the head spacing checked in mine due to the bolt working hard, but even though it was to the low side of tolerance, it was safe to shoot. I find brass cased ammo does smooth out the action over the steel cased.in any case....theoretically, if your bolt was locking in its proper position (not worn) thereby holding the casing in its proper position as not to allow gases to escape in any direction but down the muzzle...then you shouldn't have the blowback or broken cases no matter how bad the ammo is...because the only place for the gas to go is down the barrel...but in this case it's moving rearward...indicating a headspace issue from SOMEWHERE....which is why you should get the gun checked