I bought some sellier & bellot ammo for my mosin and a few of the casings cracked just like that. I think it has something to to with headspace...? I think it might be too much headspace so the shell expands and cracks...? No expert on this but I just never shot that ammo again but kt shot fine with steel cased ammo. Just a thought. I had a gunsmith tell me it has to do with the headspacing a lot of times