It could be on a legal FA lower
and then I suppose it would be ok, if the owner has the paperwork to cover it being a "sawed off shotgun", in addition to being a full auto. I do not pretend to be able to see where in their convoluted regulations the BATFE would place this, but I do believe they must have found someplace.
personally, I see nothing new or novel in this item. An interesting novelty, but belt fed uppers for the AR series are not new technology. Belt fed shotguns, and even 40mm grenade launchers are not new either.
I'm sure it was an interesting exercise for the designer/manufacturer, but what possible use could it be, other than to waste money and get stange looks?
or, of course, to post on U-tube?