The twin laws, both the one called 922(r) and Title 27 CFR 478.39 state you cannot make a semi-automatic long gun using more than 10 imported parts.
The counted parts are as quoted from 478.39 and 922(r):
(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
Hard to believe that if you use 10 imported parts you have an angelic rifle incapable of harming a flea, but if you use 11 imported parts, the firearm will become an evil baby killing device and the otherwise honest owner will become an evil maniac.
They don't make laws much stupider, and worse, they had to do it twice!
By the way, the AK (milled receiver) has 15 countable parts, AKM (stamped) has 16.