While you could have gotten away with using rock salt 50 years ago in rural areas (watermelon thieves in the South are the old stereotype for this sort of thing), those days are long gone. Nowadays you'll be arrested and sued. Don't even try it.
From a technical standpoint, shooting salt in a shotgun would probably be a bad idea, since the salt would react with the hot gases of the powder and corrode the interior of the barrel if not thoroughly cleaned soon after firing the rounds.