These ideas have been in my head for a while now, and I might have mentioned them before. For some reason, I've never written them down before. I typed this up after I read this thread on the TV Tropes fora.
Setting: Near-future, with a ton of Black Box-driven future tech
Plausibility: Possible, but might be cumbersome, unreliable, or inefficient
Pretty much exactly what it says on the tin. An auto-shotgun based on the Pancor Jackhammer that can fire three different kinds of ammo. I might discard the Jackhammer idea however, and use an original design. Possibly one with three barrels, one for each ammo type. Said ammo types are; Standard 12 gauge shell, Dragon's Breath, and slugs.
Setting: Same as above.
Plausability: Would probably work
A smallish submachine gun with a large cylindrical magazine. The main gimmick is that it's meant to be held Guns Akimbo. The recoil is low and the ammunition is caseless, so you don't need to worry about ejecting shells hitting you in the face. I even made a model in SketchUp. It also has a grip and laser sight in case you only have one, or don't have the Powered Armor it was designed for. (Said Powered Armor includes sight lines extending from the barrel as part of its HUD)