Nothing wrong with your rifle. It's the ammo.
Those commies were cheap bastards. They cut corners to extremes on production. The steel in those cases was extruded too far without being annealed, and it developed longitudinal weaknesses. If you closely examine a few dozen unfired rounds out of that spam can, you'll probably find several other rounds with the beginnings of cracks showing. If you closely examine all your fired rounds, you'll probably find several others that have cracked. That is very common. It happens to me all the time. The only noticeable problem from it is that the bolt can be a little sticky on the cracked cases. If it scares you, don't shoot any more steel cased commie surplus ammo.