Introduction
The Republican's 1989 import ban ended the import of certain semiautomatic firearms. BATFE has the authority to create administrative law to implement the Federal ban. In short, BATFE has declared that any firearm that is banned from import, cannot be legally assembled in the United States (except for Government and Law Enforcement Sales) from imported parts. BATFE identified 20 major components, and determined if no more than 10 of these 20 major parts were of foreign manufacture, then the firearm would be considered US made and exempt from the import ban.
1989 Import Ban

18 USC Chapter 44 as amended by Public Law 101-647 (enacted 11-29-90) and 
27 CFR § 178 as amended 06-25-93. 
§ 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 were approved for importation from 1989 to 1994. In 1994 BATFE changed their interpretation to eliminate the exemption for firearms with thumbhole stocks.

Imported Parts (c) For purposes of this section, the term "imported parts" are:
  1. frame, receiver, receiver casting, forging or stamping
  2. barrel
  3. barrel extension
  4. mounting block (trunion)
  5. muzzle attachment
  6. bolt
  7. bolt carrier
  8. operating rod
  9. gas piston
  10. trigger housing
  11. trigger
  12. hammer
  13. sear
  14. disconnector
  15. buttstock
  16. pistol grip
  17. forearms, handguard
  18. magazine body
  19. magazine follower
  20. magazine floorplate
FAL Specific Parts
The FAL does not have all of the parts listed. It has no mounting block, barrel extension, operating rod or disconnector. BATFE considers the FAL cocking handle to be an operating rod. The FAL has only the following potentially "imported parts"
  1. receiver (upper receiver)
  2. barrel
  3. muzzle attachment
  4. bolt
  5. bolt carrier
  6. operating rod (cocking handle)
  7. gas piston
  8. trigger housing (lower receiver)
  9. trigger
  10. hammer
  11. sear
  12. buttstock
  13. pistol grip
  14. handguard
  15. magazine body
  16. magazine follower
  17. magazine floorplate

That is 17 parts - 7 of which must be replaced with US made parts for the finished rifle to have "no more than 10" imported parts from the list. Many of the available US parts are of high quality, while some are not - see the links page for manufacturer contact information and the notes page for product reviews.