What would this abomination be under the NFA? A hand gun? An AOW?
I believe it will just be a pistol as long as there is a rifled barrel. This is how the .410 revolvers are allowed to exist.
For God's sake please buck the trend of calling the gun something that has to do with the Justice System. Please, it's getting nauseating.
I think you've got a pretty cool idea though. I'd study the saiga shotguns for how to make a reliable auto loading shot shell slinger.