Alright. So pump action it is.
We'll put a short barrel with a smart choke/muzzle brake combo that'll use a rangefinder to tighten up the spread for distance shots and open for close range carnage. Figure we can use a rotating cylindrical magazine in addition to the standard tube mag. Basically, the pumping action will also rotate the magazine, feeding a fresh round into the end of the tube every time you pump. Should give us something like 12-16 extra rounds aside from the typical 5 in the tube, one in the chamber on an 18" barrel. It'll be tip heavy, but between that and the muzzle brake, you'll get a surprising amount of control, even when shooting from the hip.
Precision trigger assembly, low friction internals so you can pump more easily, maybe a laser (your call if you want it IR or not), pistol grip and a folding stock for when you want a little more accuracy, and a free pair of earplugs, because this thing is going to be deafening in close quarters.
How's that, [member="Reverance"] ?