Erm, If it has to be a realistic weapon (looking at the posts, it won't):
A twin M16 A* which is basically two mechanisms and barrels side by side, the mags go in both mechanisms at once (both mags are joined together). There is one stock and one trigger, pistol grip and one fore-gripy-thing (similar in style to the M16 A1). It will also have an M203 and flamethrower underslung. I will call it...
'My Mutha-F*cking-Farm'
Then I can go "Here is 'My Mutha-F*cking-Farm'" and pull it out from under a poncho {Cookie for whoever knows what that is from}
* = whatever number is latest and/or best
If it can be unrealistic then it would be a toss off between a pistol (full auto) which can shoot shotgun shells and a mini mini gun. Probably the mini minigun.
The mini minigun would be the size of the LSW used by the British army, and just as light. It would be similar to an LMG: a stock, pistol grip, vertical foregrip and be as short as an LSW. It would have 6 barrels and be specially dampened to decrease recoil. There would be no electrical mechanisms so an EMP would not damage it. It would be powered by an eldritch cube of power from the 1st dimension (*dun, dun, DUNNN!*). it would be fed from an LSW magazine (or something like that) which has been enchanted with that spell from Harry Potter which Hermione puts on the bag. In effect it is the size of two or three warehouses and is full of tracer, explosive and normal rounds.
I think I'll have both...