More on pistol-gripped shotguns
From James Bardwell's always-useful list, comes a 1996 BATF opinion letter. Its conclusion is that a shotgun that starts out with a pistol grip and a rifled barrel, which is then shortened below 18", becomes a "destructive device" since it is a rifled firearm of over 50 caliber.
(I checked, BTW, and even the 28 gauge is over .50 in bore).
UPDATE: Yep, the NFA's history is interesting and at times weird. Why does it impose a $5 tax on "any other weapon," i.e., a concealable gun that is not a "pistol"? Why would Congress in 1934 be worried about gangsters with pistol canes or 19th century palm guns or smoothbore pistols?
It started out as a high tax on full autos and short barreled guns, with a $5 tax on all pistols, so as to have nationwide registration of those, too. To make sure the pistol proviso couldn't be evaded, the drafters made it pistols and any other firearm capable of being concealed.
Congress didn't buy the pistol registration, and removed it. But they neglected to remove the "any other weapon" provision. So today a pistol is not an NFA weapon, but an obscure 19th century palm gun that fires a cartridge not made in a century is NFA material.