They just sell those left over parts here. You will see it happen all the time where you can get big chunks of surplus pistons, fire control groups, and the like for cheap. There is nothing illegal about using surplus parts in your rifles (with the obvious exclusion of full auto sears, and parts considered by the BATF to be full auto parts).
What you may be getting confused with is the Title 18 U.S.C, section 922(r) law. That law limits someone who assembles a semi rifle or shotgun from having more than 10 parts maximum from a list of 20 that are original imported (not made in the USA). This is the reason why companies like Century that assemble these rifles use a certain number of American made parts.
Take an AK-47 for example. They will have 16 of the 20 items from the list. To stay compliant anyone that wants to assemble an AK-47 rifle from a parts kit will have to replace 6 or the original items with american made counterparts such as a receiver, trigger, hammer, sear, pistol grip, and muzzle device. You now have 10 or less imprted parts rom the list, and are good to go.
The parts that are pulled to replace the original ones are not illegal for you to use, as long as it doesn't push your 922(r) count past 10 imported parts.
Here is the law I am referring to-
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.
(b) The provisions of this section shall not apply to:
(1) The assembly of such rifle or shotgun for sale or distribution by a licensed manufacturer to the United States or any department or agency thereof or to any State or any department, agency, or political subdivision thereof; or
(2) The assembly of such rifle or shotgun for the purposes of testing or experimentation authorized by the Director under the provisions of Sec. 178.151; or
(3) The repair of any rifle or shotgun which had been imported into or assembled in the United States prior to November 30, 1990, or the replacement of any part of such firearm.
(c) For purposes of this section, the term imported parts are:
(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunions)
(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) Butt stocks
(16) Pistol grips
(17) Forearms, hand guards
(18) Magazine bodies
(19) Followers
(20) Floor plates
[T.D. ATF-346, 58 FR 40589, July 29, 1993]
