Problem with M-44

The other day I went target shooting with a co-worker of mine and took my SKS, Mosin-Nagant M44, and .45 pistol. He had never shot or heard an M44 go off and he wanted to see how loud they are. I fired 5 rounds from the creek bed into a dirt berm downstream before handing the rifle 5 rounds and hearing protection to Josh. He loaded the rifle as I instructed him how and fired his first shot. He then lifted the bolt without any trouble but could not withdraw it from the reciever to remove the empty brass. After a few seconds of hitting it with my palm (which really hurt) I managed to get it to open and discovered the casing had split from the opening to the first bend in the bottleneck. I assumed that the casing had split (cheap Bulgarian ammo) and had provided enough pressure against the inside of the chamber to make it difficult to withdraw the bolt, so I let him shot it a few more times. However on his second and third shot, it happened again. This time we inspected the empty casings and they were not split like #1. We decided not to shot it anymore, placed it back in my truck and moved on to the SKS. At first, I thought the problem was the fact that my ammo was stored inside my house (in the A/C) in a .50 cal ammo and swelled when exposed to the 90 degree/ 80% humidity outside air. The only problem is, the ammo can sat in the backseat of my truck for about 3 hours will we layed brick at the neighbor's house. I thought that would be plenty of time for it to adjust to the heat. Was I wrong? Would this have actually affected the ammo? It's really confusing me cause this is the first time this rifle has ever done this, I've had it about 2 years now and always used the same type of ammo. Bulgarian 7.62X54R FMJ

Could someone educate me on this. Any input would be greatly appreciated.