I had an idea. A crazy one, but it could work.
A tririfle.
A combination of a minigun, rifle and (to some extent) shotgun.
Minigun - three rotary barrels - high rate of fire
Rifle - long barrels - high accuracy
Shotgun - bullets are fed straight into the barrels, double-barreled style (speedloader would be ideal)
It could work with 3-6 barrels, but 3 seems kinda ideal, in my opinion at least.