From the evidence, I strongly suspect THAT case was fired in a 7.62x39 chamber. Look at the taper of the bulged section. Look at the diameter of the case mouth and the stepped part behind it.
I'd bet, after extraction, it was found on the ground amid other spent casings. I think the wrong case was picked up and this one did not come from the rifle in question. The primer shows severe indentation, indicative of being struck several times by a firing pin. I have used many a spent case in dry firing and in each case, the primer ends up looking just like this one. My suspicions are that whoever had to extract this case from their chamber let the hammer fall on it a few times during the process

