Some friends of mine and I were writing a fictional novel set in modern day with supernatural aspects to it, and were trying to come up with a weapon for a certain character. A handgun, to be specific. However, the fellow in question was seven foot tall with fists like ham hocks, and even an M1911A1 would look like a Mauser if you just built up the grip. So, since he needed an outsized handgrip anyway, he went to his friends to see about getting a bigger boom while they were at it.
After some discussion about the difficulties in making custom brass bigger than .50 because that's when it stops becoming bullets and starts becoming shells, and the impracticality of setting up a machine shop to produce custom brass for a single handgun found nowhere else in the world, they were looking for alternates when his eyes lit upon a 12ga shotgun shell. After some discussion, a weapon evolved. A six-chambered break-forward (like a Webley or a .38 special) twelve gage 2-1/4" revolver, double action. On paper, it seems like a practical weapon for the fellow, since his wrists and strength are well off the human charts and can handle the recoil such a weapon would produce. Slugs would... well, it's basically a .75 revolver, take a bloody guess. That's mostly for 'big game', something that NEEDS an elephant gun just to get its attention. Standard 00 Buckshot ought to do well for hall-sweeping and general usage. You can do custom loads to fit any weaknesses the monster of the week might have. And since he's working with an MIB-like organization that doesn't officially exist, 12ga shell casings are ubiquitous as hell, much moreso than custom brass that doesn't exist anywhere else in the world.
The more we started talking about it, the more it sounded like it might actually be possible to build for real. It doesn't have a stock, so it wouldn't fall under SBS in most jurisdictions (except maybe California... those guys are nuts), it wouldn't fall under breeching weapon either, so probably just get lumped into AOW. Obviously no human would be able to fire the damn thing without breaking their wrist, but I was wondering if it might be possible to build. It'd probably need an extended barrel (for a pistol) to make sure enough of the powder burns before it leaves to have the muzzle velocity you'd want, which might hamper quick-draws. Shotgun shells are pretty low on the bore pressure so the classic problem involving the revolver chambers would be less of a problem. It would almost have to be smoothbore, which means its accuracy outside fifty feet is pretty much crap, but would further reduce bore pressure issues.
I dunno, from an engineering perspective, you think it would be feasible? I bet Hellboy wouldn't mind one...