We are getting off track here - remember the legal definition of a shotgun:
(18) "Short-barreled shotgun" means a shotgun having one or more barrels less than eighteen inches in length and any weapon made from a shotgun by any means of modification if such modified weapon has an overall length of less than twenty-six inches.
(19) "Shotgun" means a weapon with one or more barrels, designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned, made or remade, and intended to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.
This is important because 18 year olds can buy shotguns and rifles, for any thing else classified as a gun, the age is 21. Obviously, this ruling that pistol grip weapons without shoulder stocks with shotgun barrels was a ruling to prevent 18 to 20 year olds from buying this type of weapon.
Now it gets interesting. Look at the legal definition of a destructive device:
The term "destructive device" means (1) any explosive, incendiary, or poison gas (A) bomb, (B) grenade, (C) rocket having a propellent charge of more than four ounces, (D) missile having an explosive or incendiary charge of more than one-quarter ounce, (E) or (F) similar device; (2) any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter, except a shotgun or shotgun shell which the Secretary finds is generally recognized as particularly suitable for sporting purposes; and (3) any combination of parts either designed or intended for use in converting any device into a destructive device as defined in subparagraphs (1) and (2) and from which a destructive device may be readily assembled. The term "destructive device" shall not include any device which is neither designed nor redesigned for use as a weapon; any device, although originally designed for use as a weapon, which is redesigned for use as a signaling, pyrotechnic, line throwing, safety, or similar device; surplus ordnance sold, loaned, or given by the Secretary of the Army pursuant to the provisions of section 4684(2), 4685, or 4686 of title 10 of the United States Code; or any other device which the Secretary finds is not likely to be used as a weapon, or is an antique or is a rifle which the owner intends to use solely for sporting purposes.
Thus, the larger diameter weapons are destructive devices unless:
1-a shotgun with a suitable sporting purpose
2-rifles with a sole sporting purpose
3-antiques
4-things that arent weapons (either by design, or by likely use)
4-weapons redesigned to be used signaling/line throwing/etc
Which "out" does pistol grip only smoothbores fit? They have already been declared not to be shotguns, so (1) is out ..........
And all destructive devices have to be registered as NFA weapons. This is the issue.