Never actually tried it, but my WAG is that it would be mostly gimmick. The detonation sending the rock salt through the barrel would likely fracture the salt into smaller pieces. Than and the irregular surface wouls give extremely poor aerodynamic as well as ballistic performance. Very short effective range and little practical value, IMO. Having done rice in my BB Gun barrel as a kid, shooting at lizards and such, performance was crappy at best and inconsistent. If you are looking to be eco-friendly with rock salt over conventional shot, you aren't thinking about your defense capabilities very practically. Otherwise, rock salt would easily be defeated by most clothing anyway (lack of mass as well as the density of projectile material).
I am not a scientist nor did I sleep at a Holiday Inn Express last night. Just sayin' that it would make a lot of noise and the effective range seems a waste of effort compared to many other options in the SD/HD realm.