Sorry for the double post, but here's the link to the study they are talking about.
http://atf.gov/publications/firearms/012611-study-on-importality-of-certain-shotguns.pdf
What's being talked about is this particular portion the conclusion:
The purpose of section 925(d)(3) is to provide a limited exception to the general prohibition on the importation of firearms without placing “any undue or unnecessary Federal restrictions or burdens on law-abiding citizens with respect to the acquisition, possession, or use of firearm. Our determinations will in no way preclude the importation of true sporting
shotguns. While it will certainly prevent the importation of certain shotguns, we believe that those shotguns containing the enumerated features cannot be fairly characterized as “sporting” shotguns under the statute. Therefore, it is the recommendation of the working group that shotguns with any of the characteristics or features listed above not be authorized for importation.
So what are the characteristics that they are talking about?
Here's one from the report:
The fact that a firearm or feature was initially designed for military or tactical applications, including offensive or defensive combat, may indicate that it is not a sporting firearm.
Here are the list of features that according to the study make it not for sporting:
(1) Folding, telescoping or collapsible stock.
(2) Bayonet Lug.
(3) Flash Suppressor.
(4) Magazine over 5 rounds, or a Drum Magazine.
(5) Grenade Launcher Mount.
(6) Integrated Rail Systems.
(7) Light Enhancing Devices.
(8) Excessive Weight.
(9) Excessive Bulk.
(10) Forward Pistol Grip or Other Protruding Part Designed or Used for Gripping the Shotgun
with the Shooter’s Extended Hand.
What does that make it? A self-defense shotgun.