Here's the actual text from the section regarding parts count requirements:
[Code of Federal Regulations]
[Title 27, Volume 1]
[Revised as of April 1, 2002]
From the U.S. Government Printing Office via GPO Access
TITLE 27--ALCOHOL, TOBACCO PRODUCTS AND FIREARMS
CHAPTER I--BUREAU OF ALCOHOL, TOBACCO AND FIREARMS, DEPARTMENT OF THE
PART 178--COMMERCE IN FIREARMS AND AMMUNITION--Table of Contents
Subpart C--Administrative and Miscellaneous Provisions
Sec. 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.
(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
(3) Barrel extensions
(4) Mounting blocks (trunions)
(5) Muzzle attachments
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
[T.D. ATF-346, 58 FR 40589, July 29, 1993]
The parts highlighted in red do not apply to standard AK-type rifles:
A stamped rifle with a muzzle device has 16 countable parts.
A stamped rifle without a muzzle device has 15 countable parts.
A milled rifle with a muzzle device has 15 countable parts.
A milled rifle without a muzzle device has 14 countable parts.
Muzzle nuts do not count as a muzzle device. Refer to the second letter in this link:
Since a muzzle nut does not function as a flash hider or grenade launcher, it is not a countable part.
A trunnion is the block that is used to mount the barrel to the receiver. The stock tang or rear block for a folder is NOT a trunnion. Referring to these pieces as trunnions is incorrect. All BATFE references to "trunnion" are used to describe the attachment point of the barrel to the receiver. The BATFE homepage has information regarding the importation of rifles for unrestricted sale. In these documents, the trunnion is destroyed by cutting with a torch. In these documents the trunnion is the point where the barrel is attached to the receiver.
The parts count requirement does not apply to pistols.
For AMD-63/65 rifles:
Replacing just the front pistol grip with a U.S. made copy does NOT affect the parts count. The entire front grip including the stamped metal handguard AND the pistol grip must be replaced to count as a single U.S. part.
Using a barrel extension such as the Tapco piece adds another part to the total count. Because the Tapco extension is a U.S. made piece, it effectively cancels itself out of the parts count total. If the original AMD-65 muzzle brake is used it still counts on the list as an imported part. This means that even if a Tapco barrel extension is used, there are still a total of 6 imported parts that must be replaced to be legal.
Using a U.S. made combination brake/extension solves 2 issues. The extended brake makes the barrel length meet the legal requirement. The combination device is only considered a muzzle device because it is a single piece. This means only 5 other parts need to be replaced with U.S. copies.
The easiest way to check your parts count is to subtract your U.S. parts from the list and count the remaining listed parts. If the number is 10 or less, you're golden.