If legal, 12GA double-barrel shotgun pistols (RECOIL central) are probably muzzleloaders which is why you probably found one for sale. I discovered this oddity after being interested in the Taurus Judge .410 revolver.
That only works for guns that can be chambered for a round that was designed from the get go to be used in a rifle or pistol. The .410 can have the bore enlarged and rifled for the .45 Colt round is why you can have a .410/.45 pistol. Nobody ever made a .729 magnum round for pistol or rifle so 12 gauge doesn't quite work out like a .410/.45 otherwise a rifled slug barrel on a short barreled shotgun would be legal without the tax stamp.Unless you rifle the barrel..
Provided you could find somebody to do it. I doubt any self respecting gunsmith would touch it.Unless you rifle the barrel..
Stevens and some others made short barreled centerfire shotguns with pistol grips until 1934 but the national firearms act of 1934 made them illegal.
I came upon a HandyGun..I sent the barrel off to a reputable gunsmith He rifled it cost $35 .Provided you could find somebody to do it. I doubt any self respecting gunsmith would touch it.