"Normally it cracks because some fool used de-linked machine gun ammo in it."
I don't know of a single machine gun that has ever used linked 7.63x25mm.
All of the submachine guns that I know of that use this, or 7.62x25mm Tokarev ammo, are magazine fed.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.