I have uses coco wheats in a 38 to get bats out of a barn. It just stuns them and you don't have to patch the roof. But I don't think rock salt in a shot gun is a good idea.
and
There have been times I've wanted to shoot my horses doing bad habits in the middle of the pasture. Paintballs would either welt at close ranges, or miss at farther. Throwing rocks was the most effective even if 90% missed. Would be nice to have a light load of salt for the naughty animals knowing it prob wouldn't penetrate at 40-75 yard but would definitely stop the bad activities with noise and touch sensations.
Are the main kinds of things I'd think rock salt loads would be good for. Scaring away animals, rather than people. The salt would add some teeth to the noise, since otherwise animals will quickly learn that all that banging is just a noise. At the local outdoor range, the songbirds and groundhogs aren't perturbed one bit by all the gunfire (although they may be deaf by now!).
Anyway, just driveway salt. Make sure you put it in a wad that will prevent the salt from touching the inside of the barrel. Clean the gun out good whenever you shoot them.