12 bore muzzleloading SxS set of barrels would work into a pistol and that would be doable/legal here, at this time, if registered as a handgun (restricted firearm).

I would think that a 12 bore ctg. gun might not be quite legal - be classified as a cut-off shotgun, even if pistol gripped, due to readily available ammunition.

A set-back handle would allow the "long-arm" locks, and yet still have the pistol grip. Would look "off" though.