I stand by my original statement...
From the ATF website...
18 U.S.C. 922(r) makes it unlawful for any person to assemble from imported parts any semiautomatic rifle or any shotgun which is identical to any rifle or shotgun prohibited from importation under section 925(d)(3) of the GCA. Regulations implementing the law in 27 C.F.R. 178.39 provide that a violation of section 922(r) will result if a semiautomatic rifle or shotgun is assembled with more than 10 of the following imported parts:
(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
Section 922(r) does not prohibit the importation, sale, or possession of parts which may be used to assemble a semiautomatic rifle or shotgun in violation of the statute. However, 18 U.S.C. § 2 provides that a person who aids or abets another person in the commission of an offense is also responsible for the offense. Therefore, a person who sells parts knowing that the purchaser intends to use the parts in assembling a firearm in violation of section 922(r) would also be responsible for the offense.
It should be noted that a violation of section 922(r) may result even if the assembled rifle or shotgun does not meet the definition of "semiautomatic assault weapon" in the GCA. For example, a person who assembled an SKS rifle with a folding stock and fixed magazine would violate section 922(r), since the SKS would have more than 10 of the imported parts specified in the regulation. However, the SKS rifle with a fixed magazine would still not be regulated as a "semiautomatic assault weapon."