Did you check the diameter of the bullets used that resulted in the keyholing? As an experiment take a loaded round and stick the bullet into the muzzle end of the barrel. How far does it go in? Try this with both types of ammo. If the bullet goes in all the way to the brass, then something is wrong. The ammo may be incorrect. Can you measure the bullet diameter with a digital calper? If your Mosin shot well before, then it is unlikley that the rifle is the problem.