In order to fully understand 922(r), you have to first understand 925(d)(3). You see, 925(d)(3) bans the importation of "semi-automatic assault weapons" which are defined as semi-automatic rifles and shotguns with certain features such as pistol grips, bayonet lugs, flash hiders, etc. 922(r) applies only to imported firearms which, after their importation, are reconfigured in a way that would be banned from importation under 925(d)(3). This is why guns like Saiga rifles and shotguns are only subject to 922(r) when modified in certain ways: in their stock configuration they have no "assault weapon" features and thus are allowed for importation under 925(d)(3).
922(r) contains a list of certain parts including recievers, stocks, triggers, hammers, disconnecters, handguards, and barrels. Under 922(r), an imported gun is illegal if reconfigured with "assault weapon" features unless no more than 10 of the specified parts are imported. This is why you see a cottage industry for easily replacable U.S.-made parts for imported firearms like AK-47 derivatives and SKS rifles. Also, it is worth mentioning that firearms imported prior to 1989 (when both laws took effect) are grandfathered and thus not subject to 925(d)(3) or 922(r).
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