Basically, it works like this - by law, any firearm with a bore greater than 0.50" in diameter (i.e. most shotguns, imported or domestic) is a destructive device that requires registration under the NFA. Shotguns are exempted from this IF the Attorney General determines that they have a "suitable sporting purpose."
One of the problems is that while we know what the "sporting purposes" test for imported rifles and imported handguns are, there is no test for imported shotguns. The stated purpose of this study is to establish a "sporting purposes" test for shotguns. This study takes the view that a wide range of accessories for shotguns do not have a suitable sporting purpose.
This is a big concern because once it doesn't have a sporting purpose for importation purposes, it doesn't have a sporting purpose for domestic purposes either. And in the few examples in the past when the ATF banned a shotgun from importation under the sporting purposes test, they also made possession of the domestic equivalent of that shotgun subject to NFA regulations.
The particularly scary result of this is that the extension tube on my 18" Remington 870 would not only turn my otherwise plain pump shotgun into a destructive device; but even if I removed the magazine extension to avoid the rule, I would still need to register the extension itself or destroy it to avoid violating the NFA. And constructive posession rules would be a nightmare... if a flashlight makes a shotgun an NFA weapon, than the same nylon buckle and Surefire G2 I have for my AR15 (completely legal) could be constructive possession of an NFA weapon if I also own any shotgun with a bore greater than 0.50".
Hell, given past ATF rulings on shoestrings, owning a roll of duct tape, a shotgun and a flashlight all together might be constructive possession of an NFA weapon.