Have you ever actually shot rock salt? I have, and it works when loaded in a shot cup at around 700-800fps. I mentioned it as his intent was as pest deterrent, not a kill shot. Having said that, he can also buy as small as #12 lead shot.
Yep, it works for the intended purpose. It's not supposed to be capable long distance.
To the OP's question - I have loaded 12 gauge shotshells with plastic airsoft BBs. It can work fine as a pest deterrent, such as teaching the neighbor's mutt to stop pooping in your yard, but it's not effective at killing anything if that's your goal. I've used them most at basically airsoft velocity (you can see them in flight) with very small charges of fast powder; that can make a very quiet load. Removing the cushion from the wad helps a lot with consistency, since those plastic pellets don't offer much resistance, and it increases capacity by quite a bit as well. The terminal effect is essentially just like a bunch of simultaneous hits from an airsoft gun - it'll leave some red welts on bare skin but that's about it, except at very close range.
