People have wondered about a slug-firing shotgun for absolutely ages. It never made it in, as it crosses into other weapon's territory too much. But barrel safe ammo sounds great. Little/no damage (maybe a 1d1 attack?), just enough to get monsters coming at you, but unlikely to blow a barrel up unless you shoot it a lot. Bird-shot assembly for shotguns? TA assembly for any shotgun, drops damage to 1d1. How would drop-off work with that? Would it still do 1 damage at max range? Or would it have to be more like 3d1 or higher to stop it getting rounded down too early (thus giving it almost no range?). Still a lot safer than a normal shotgun's blast.
Paintball ammo sounds good for a non-shotgun style weapon, but more as a gimmick. Paint the monster blue (in ascii, just add a blue graphics effect on G mode), drop their accuracy by 20% for 5 turns, 1d1 damage, enrage them. I don't even know if monster accuracy it stored individually in memory, or just as a reference table look-up for any particular type of monster, so it might be annoying to implement, It sounds fun, but I don't know if I'd every use it. Less monster accuracy, or just fire my most powerful weapon at them. Let's see............ I don't think we really need any other gimmick weapons that are highly situational/largely-useless.
Bird-shot would be very handy if it gets around the drop-off of shotguns. A safe radar shooter would be excellent, and maybe even overpowered with Juggler or Int(2). Maybe make it a special instead of an assembly (I'd still keep it and use it in most builds. So very handy). Really easy to implement, so-much-so that I'll make up a mini-mod today to test it for range and barrel popping.
Edit: I always get messed up with shotgun drop-off. It's linear, so a 7% drop-off shotgun will always get 14 squares range. Which makes 1d1 still work fine (any damage under 1=1 damage). So for this specific weapon, 1d1 damage would be more powerful than say 3d1, because it would give less chance of popping barrels close up, even if 3d1 would do an extra point of damage to things close to you. That's not really what this weapon is for anyway. I hope we do get a bird-shot assembly, or a special or unique. I'd call it the "Weak-sauce" and let players work out how horribly over-powered it is. A 3d1 shotgun? Why? Then you realize how good the thing is with the game's mechanics. :)