§922(v)(1) It shall be unlawful for a person to manufacture, transfer, or possess a semiautomatic assault weapon. (2) Paragraph (1) shall not apply to the possession or transfer of any semiautomatic assault weapon otherwise lawfully possessed under federal law on the date of enactment of this subsection (09-13-94)
§921(a)(30) The term "semiautomatic assault weapon" means
(A) any of the firearms or copies or duplicates of the firearms in any caliber, known as - . . .
(v) Fabrique National FN/FAL, FN/LAR, and FNC.
(B) a semiautomatic rifle that has the ability to accept a detachable magazine and has at least 2 of
(i) a folding or telescoping stock
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon.
(iii) a bayonet mount
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor
(v) a grenade launcher
§ 178.39 Assembly of semiautomatic rifles or shotguns.
(a) No person shall assemble a semiautomatic rifle or any shotgun using more than 10 of the imported parts listed in paragraph (c) of this section if the assembled firearm is prohibited from importation under section 925(d)(3) as not being particularly suitable for or readily adaptable to sporting purposes.
925(d)(3) includes FN FAL (L1A1) style semiautomatic rifle with folding stock, military style stock with separate pistol grip, threaded muzzle, flash hider, grenade launcher, bayonet lug, or night sights.
Note: FN FAL (L1A1) style semiautomatic rifles having a thumbhole style stock without the above features are approved for importation.
(c) For purposes of this section, the term "imported parts" are:
(1) Frames, receivers, receiver castings, forging or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (turnings)
(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) (Magazine) Followers
(20) (Magazine) Floorplates
(1) Receiver
(2) Barrel
(3) Muzzle attachment
(4) Bolt
(5) Bolt carrier
(6) Gas piston
(7) Trigger housings (lower receiver?)
(8) Operating rod [sic]
(9) Trigger
(10) Hammer
(11) Sear
(12) Buttstock
(13) Pistol grip
(14) Handguards
(15) Magazine body
(16) Magazine Follower
(17) Magazine Floorplate
That is 17 "evil" parts - 7 of which must be replaced with US made parts. DSA offers by far the best US parts and First Son Enterprises (sold through RPB) offers the second best. There is no third best - the others are poor quality and dimensionally off (see parts evaluation for details).
To be compliant, you cannot use a flash suppressor, so consider the following combinations
This combination is for building a metric rifle and allows use of an imported magazine.
US receiver
US muzzle brake
US gas piston
US pistol grip
US trigger
US hammer
US sear
This combination is for building a metric rifle and is least expensive.
US receiver
US muzzle brake
US gas piston
US pistol grip
US trigger OR US hammer OR US sear
US magazine floorplate
US magazine follower