Find a Penn Arms or Sentinel Arms Striker-12 shotgun and do the paperwork for it (they got ruled 'Destructive Devices' because of faggotry). $1000 to $2000 or so.
12-Gauge, 12 shells. Some are short like this which is handy, yet they retain their high capacity compared to shotguns with tube magazines. It's basically a big revolver, a double-action one, so you can shoot it pretty quickly by just pulling the trigger fast. As the cylinder is indexed by a spring mechanism you wind up when loading the chambers, the gun is not sensitive to the power of ammo like an automatic is, and since it's a revolver where the shells only sit in the chambers, you could keep it loaded, something which can cause problems for shotguns using detachable drums or box magazines (where the plastic of the shells can slowly squish from the the magazine spring).
The main drawback here is A), the NFA bullshit, B), since it's a revolver you'll kind of feel some gas vent out from the cylinder housing from the gap between the cylinder and the barrel, and C), the reloading would take a while.
However, it's going to be cheaper and much better suited to home defense compared to a .50BMG rifle, just deal with the mild discomfort, and finally, 12 shotgun shells is a LOT, most pumps and automatics max out 8+1 or so.
Combine with a mace/OC spray system and a gasmask.