I've heard various stories, mainly in fiction, of loading a shotgun with rock salt, with the intention of inflicting serious pain on an intruding person or animal non-fataly. But, apart from the real danger of blowing up the gun that midchuck mentioned, including nails would be particularly vicious. There would be no way to say it couldn't wound fatally, and someone could very well die in much worse agony than if the salt wasn't included.