BATF has also pretty much stated that unless you work magic with the feed-mechanism, it's probably illegal to make them as a "shotgun" anymore.
-Destructive devices.

So since it's already lumped in the same legal-pile as a belt-fed grenade-launcher...
-Remember kids, so long as it uses that old M16A1 lower from 1972, it's still legally just an upper.

I'm thinking 57mm auto-cannon... [url=https://www.youtube.com/watch?v=geGtS0157_0]or retrofitted to an automatic gun-mortar, 82mm[/url].