It's filled with rock salt - rock salt is like shrapnel (sp?) it's not like a bullet that's gonna go straight in, it breaks into pieces and they stay closer to the surface. Plenty of people have been shot by rock salt, and I believe dying from a wound like that is rare unless maybe the gun was right up against you.
Shotgun Blast
Remember when Budd shoots The Bride, whatever happened to the shotgun blast into her chest. It seems it only affected her in one scene and then it seems as if it kind of didn't matter anymore. from being shot in the chest, she is able to be burried and break out of a coffin completley disregarding the shotgun wound.