According to the ATF's own definition of sporting shotguns.
If you've read the study
, they state those criteria very clearly. They specifically state they see no difference between detachable magazines and tube magazines... detachables don't swap out as fast as some think and tubes are faster to reload than many might believe.
The list of criteria (beginning page 14 of the .pdf):
1) folding/telescoping stock
2) bayonet lug
3) flash suppressor
4) magazine larger than 5 rounds or a drum
5) grenade launcher mount
6) tactical rails
7) light enhancing devices (basically, night sights)
8) excessive weight
9) excessive bulk
10) forward grip (not a handguard, but an actual protruding grip)
As imported, there's no substantive difference between a Saiga 12 and any other semiauto shotgun like a Beretta 391. Sure, the Beretta is prettier and probably better for clays, birds, etc., but that isn't the ATF's concern.
About the only gun I know of that might be affected by the study right now is the Chinese copy of the 1897 Trench Gun (thanks to the bayonet mount).
My only concern is what happens to modified imported shotguns- while we've generally acted like 922r applies to shotguns, they aren't necessarily covered there. It is possible that the ATF could be *really* problematic and treat imported shotguns modified to an unsporting configuration as destructive devices or some other restricted firearm. Or they could grandfather them. Or they could just say comply with 922r parts counts.