I did some research on this very topic less then a month ago. I used my 2000 edition FEDERAL REGULATIONS REFERENCE GUIDE and spoke to lawyer/importer that brings firearms and parts in on a regular basis from Russia. Import regulations can be found starting on page 35.
Such things as firearms, parts and high capacity magazines are imported with an approved Form 6. There is also an importer license that is issued by the ATF for the purpose of importing such items. A dealer may also submit a Form 6 for approval in limited cases. Such as for the purpose of his personnel collection or an item for a customer that is a resident of his State.
27 CFR Part 47 describes what items must come in with an approved Form 6. There is also a part of this section that describes bringing in limited items with no Form 6. Here is most of the text that would be interesting to us;
27 CFR Part 47 - Importation of Arms, Ammunition and Implements of War
47.11 Meaning of terms
Defense Articles
Any item designated in 47.21 or 47.22. This term includes models, mockups, ......
Firearms
A weapon and all components and parts thereof, not over .50 which will or is designed to or may be readily converted to expel a projectile by the action of an explosive, but shall not include BB and pellet guns......
Machine Guns
........is a firearm originally designed to fire, or capable of being fired fully automatically by a single pull of the trigger.
47.21 The US Munitions Import List
CATEGORY 1 - Firearms
(a) Nonautomatic and semiautomatic firearms, to caliber .50 inclusive, combat shotguns, and shotguns with barrels less then 18 inches in length, and all components and parts for such firearms.
(b) Automatic firearms and all components and parts for such firearms to caliber .50 inclusive.
47.22 Forgings, castings, and machined bodies.
Basically just because an item is not completely machined you still need a permit..."except for such items as are in normal commercial use."
47.41 Permit requirement
(c) A permit is not required for the importation of......(2) minor components and parts for category 1(a) firearms, except barrels,...receivers or complete breech mechanisms, when the total value does not exceed $100.00 wholesale in any given transaction.
Conclusions:
Seems to be according to 47.41 semiauto parts can be brought in without an approved Form 6 in small lots of less then $100.00 in value per transaction (excluding barrels receivers and breech mechanisms). If someone was wanting to do business in imported firearms parts they would need to get an importers license and do everything on a Form 6.
BTW I just got a letter in the mail last week saying that barrels are okay as long as they are for replacement.