Also, see the main 922(r) page
ATF's first test of "sporting" vs "non-sporting" firearms was with the USAS-12 shotgun. The Gilbert Equipment Company wished to import it, and took it all the way to court. This saga lasted from 1986 to 1989, and in the end, the ATF had more or less decided how it wished to determine whether or not a rifle is "sporting".
Purposes Recognized as "Sporting" by ATF
In the 1989 Gilbert case, ATF stated that hunting and skeet shooting are considered to be sporting purposes for shotguns.
Purposes Disallowed as "Sporting" by ATF
"Police combat competitions" or "Police combat games" (most likely taken to include any tactical competition) are not considered to be sporting applications.
The determination of whether or not a rifle is "sporting" does not solely hinge upon the presence or absence of the following features. However, the fewer features it has on this list, the more "sporting" it is. Furthermore, unless you wish to write to ATF for specific approval of one configuration or another, do not add anything on this list to a foreign-made rifle unless you have first converted it to have 10 or fewer imported parts.
The 'features' are:
Ability to accept a large capacity magazine
Pistol grips (not to be confused with thumbhole stocks, even Dragunov-style ones)
Ability to accept a bayonet
Flash suppressors / hiders - the "capacity to accept" tends to be invoked here, which is why threaded barrels aren't common to find on imported guns.
Integrated, military-style bipods
About Pistol Grips
Although state assault weapon laws tend to view thumbhole stocks just as restrictively as pistol grips, it is important to bear in mind that 922(r) is Federal, and it's a very different set of rules.
There is a large amount of vagueness in 922(r) around bipods. By "military-style bipods", ATF means integral bipods. Harris-style or clamp on bipods are not covered by this prohibition. ATF is referring to, for instance, the wide "military forearm" for the H&K 91 / G3 platform, which includes an integrated bipod.
A friend received an email from ATF's Firearms Technology Branch, on 5/24/07, when inquiring about the legality of installing a Dragunov stock on his rifle: "The pictured Dragunov type stock or any “thumbhole type stock” can be added to an SKS. By doing this, the firearms is no longer in its original military configuration, however, it does not make the firearm “non-sporting”. Therefore, adding the Dragunov stock to an SKS is not a violation of 18 U.S.C. 922(r) and is acceptable under Federal law." This, by the way, is consistent with their stated position on Monte Carlo stocks.
In short, installing a "sporting" stock does not seem to require the removal of other non-sporting features.
Although I have no ATF letters to confirm this, it would appear that the "capacity to accept a flash hider" on a threaded barrel can be effectively neutralized by tack-welding an accessory onto the barrel. I'm fairly sure that welding it on neutralizes the "muzzle devices" point of the countable 922(r) checklist too. When you weld an imported accessory onto an imported barrel, they become one counted part for 922(r) purposes, and not two. I have seen plenty of rifles from Century Arms with slant brakes tack-welded over threaded barrels. Cutting the weld will definitely restore the ability to accept a standard barrel accessory, however after this alteration the rifle does need to be audited for its 922(r) compliance parts count.
A particularly fascinating read on 922(r) is the Gilbert case.