Shotguns are a \'special case\', and if the powers that be determine that a particular one isn\'t \'sporting\', away it goes into DD land (see: USAS-12, Street sweeper, et-al)
I would not bet a lot on a determination one way or the other for a Gatling shotgun. Not a lot of skeet fields that would let you shoot one.
Black powder muzzle loaders are somewhat better off.