The only 'logic', in order to simply call it that, is usually a shotgun doesn't have a rifled barrel. Makes it simple for that Goobermint; no rifling, it cannot be a rifle. Nonetheless you will find rifled shotgun barrels or slugs that could manage the spin and they're lawful for http://www.askmap.net/location/7181593/united-states/hand-crafted-metalworks