You can use no more than 10, specific, imported parts to assemble a rifle after 1994 (I believe).
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>LESSON 901: TEXT OF RELEVANT "RULES"
(Updated 03-28-99)
§ 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.
(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
[T.D. ATF-346, 58 FR 40589, July 29, 1993] [/quote]
This rule is the reason your FAL has a thumbhole stock and, in most likelyhood, no muzzle device of any kind.
You can replace some of the "bad" imported parts with those that are made in the USA and "ATF Approved" to remain legal. Typically, these parts are: muzzle brake (brazed or welded in place), trigger, sear, hammer, pistol grip, gas piston, follower, floorplate, and sometimes the handguards.
I found the above quote from the FAL files at:
http://www.fnfal.com/falfiles/index.html