Here's how it works. When a rifle is imported, ATF determines whether or not it is a "sporting" firearm. This is because the 1968 Gun Control Act deemed only "sporting" firearms (plus C&R firearms) could be imported for sale to civilians.
922(r) says that you cannot assemble from imported parts, a firearm that is not legal for importation. This basically prevents importing all of the parts and assembling a firearm that failed to meet the "sporting" clause and was denied importation in its fully assembled configuration.
The problem occurs when someone purchases a rifle and decides they want to make a few alterations to the firearm. The most common one is someone wanting to put a folding stock on a SKS. A SKS with a folding stock does not meet ATF's criteria for a "sporting" firearm and is not legal for importation. If you take your SKS that is already in the country and add a folding stock, you are violating 922(r).
ATF now turns around and says there is a way for you to legally add that folding stock. They say you can change a specific number of imported parts on your rifle with parts made in the US and it suddenly becomes "Made in America" instead of being an import. Firearms made in America do not have to meet this "sporting" clause, so just about any modification is legal.
To make this sudden re-birth as born in the USA, ATF has made a list of 20 parts that they consider major components of a firearm. You take the list and determine how many of the listed parts your rifle contains. It might be 13 or 16 or 19. Whatever.
ATF then says that if your rifle doesn't contain more than 10 "imported" parts, it is not considered an imported firearm. If you figured out your rifle had say 16 parts on the list, you replace any 6 parts on the rifle with US manufactured parts and your rifle is now "Made in America".
You can now make any modifications to your rifle you desire that are legal for any other US made rifle.
The parts list is:
Quote:
(1) Frames, receivers, receiver castings, forgings
or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunnions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates