Per NFA'34, a firearm with a bore over ½" is a "destructive device" unless the Treasury Secretary deems it suitable for "sporting purposes". A 28ga shotgun should be chokable down to 0.499" without any problem; a 20ga or 12ga would require a larger bore.
But I doubt they will ever have the guts to flat outlaw 12 ga shotguns. - Bullpup 'style' sure, they will no doubt try.