Section 922(r) of the Gun Control Act (GCA) prohibits assembly of
certain semiautomatic rifles and shotguns from imported parts. The
implementing regulations in section 178.39 of Title 27, Code of
Federal Regulations provide that no person shall assemble a
semiautomatic rifle or any shotgun using more than 10 of the
following imported parts if the assembled firearm is prohibited
from importation under section 925(d)(3) of the GCA as not being
particularly suitable for or readily adaptable to sporting
purposes:
(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
(

Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
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(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers