Salt causes rust. That's a given.
Another problem with salt... it's hygroscopic. That means it soaks in water out of the humidity in the air. When it does that it turns into clumps. With rock salt there's always the possibility you'll wind up shooting the salt equivalent of buckshot or slugs, and killing someone or something. We say over and over again - anything, and that means ANYTHING, fired out of a shotgun CAN BE LETHAL, and that includes even blanks.
Forget about rock salt, or the idea of ANY "nonlethal" or "less lethal" or any other kind of rubber buckshot shotgun shell or the like. No matter what it's loaded with, firing a shotgun constitutes use of lethal force. You as an armed citizen need to be completely legally justified in using lethal force if you shoot ANYTHING out of a shotgun at another human being.
And forget about the other "novelty" stuff too, unless you just want more range toys to play with.
"Bird bombs" or cracker shells work well for their intended purpose, scaring away birds or other nuisance critters. There was a news story last week, though, about a guard at an oil field on the North Slope in Alaska who got in trouble with the Federal government for mistakenly shooting a nuisance polar bear with a cracker shell instead of the bean bag round he had intended to use, and killing it. Once again - ANYTHING fired out of a shotgun can be lethal.
Flares are meant for flare pistols or in a pinch, CYLINDER CHOKED riot barreled shotguns. Flares occasionally get stuck in chokes in shotgun barrels, and if one does then you get to watch the end of your shotgun barrel glow red as the stuck flare burns itself out in your barrel. Probably don't want to do that...
"Dust busters" or breaching rounds are intended for that exact purpose - breaching doors, by blowing off locks and/or hinges while posing a reduced level of danger to anyone inside the building. They work pretty well for that. But I have been told for a fact that they too are lethal if used against a human being. Again, they're something not really useful to the ordinary armed citizen, who isn't likely going to have a string of well trained friends on hand to set up a stack, or a legitimate reason to breach a door and clear a room.
There's a lot of other stuff out there besides the usual birdshot, buckshot and slugs. There's at least one company making a 'buck and ball' round in 12 gauge, they also make a load that fires two big round 'punkin balls' at once. Another company makes a 3" load with three bore size hard lead punkin balls. If I were packing a marine finish pumpgun on board a boat offshore, I'd have to do some experimenting with those loads.
There are more and more different kinds of slugs showing up on the market. One newer offering from Europe is made of steel, which might be useful for big game hunters in areas where lead projectiles are not allowed. And at least one US company is offering a sabot slug made of copper, which would meet the same lead-free requirement.
A certain number of folks have ideas about getting a shotgun bandolier, and loading it up bandito style with two or five of every different kind of shotgun shell out there "just in case they need" whatever. Well, it might sound good in theory but chances are it isn't going to work out well in practice. It's pretty easy to keep things sorted out just kicking around on the flat range. Under pressure though, who knows what's likely to get stuffed into the shotgun. Find a way to carry extra ammunition that keeps it clearly segregated and easy to find what you want without having to look, and if you choose ammo that lets you easily FEEL the difference (star crimped buckshot and roll crimped slugs, for instance) so much the better.
Remember that security guard and the polar bear? Murphy loves ya, baby.
Best approach to shotgun ammo is KISS - keep it simple and sure. Narrow things down to a manageable assortment of loads, two at the maximum - one kind of buckshot and one kind of slug, for example. Keep buckshot only in the magazine and slugs only in the butt cuff or Sidesaddle, if your situation allows those choices of course. Or just use slugs and be done with it, again if your situation allows it. Otherwise choose a different kind or kinds of defensive ammunition, based on your particular circumstances. Don't 'candy cane' or 'dutch load' your magazine, because you need to know for certain sure what will be coming out of that muzzle when you press the trigger.
Or not...
fwiw,
lpl
These are no-nos.