From much research into the subject Ill say this, american weapons as far as semi-auto rifles or shotguns are concerned are defined as having 10 or less foreign parts according to Legislation from 1990.
Semi-auto rifles and shotguns with more than 10 imported parts according to the list are legally considered foreign and are subject to 922 legislation compliance.
922 compliance really was about not having what the ATF termed unsporting features such as pistol grips, bayonets, grenade launcher,....etc
So by 1990 semi-auto foreign rifles and shotguns could no longer be imported with traditional battle features as they were throughout the 1980's.
All semi-auto foreign rifles and shotguns on American soil prior to the cutoff date that the legislation took effect in 1990 would be grandfathered as preban(1990) and would remain legal to own. However, any new imports would have to become compliant to the new law. Chinese exporters continued to send 100 percent foreign ak-47s without the uncompliant features.
Examples: pre 1990 sks's were allowed in with bayonets regardless of C&R status.
pre 1990 ak-47's were allowed in with bayonet, pistol grip, and folding stocks.
Any semi-auto rifle or shotgun imported after the 1990 cutoff date could not have the ATF defined unsporting features and could not be modified into a nonimportable rifle or shotgun unless it was made 922 compliant by reducing the total ATF listed foreign parts to 10 or less.
C&R imports bypass the import law if they are kept in original configuration.
Once, the C&R import is modified outside of original configuararion it is subject to 922 compliance. Example of this is the YUGO SKS.
A YUGO SKS if modified such as changing the wood stock to a plastic sporter
stock suddenly falls under 922 and without removing the unsporting features such as a bayonet, grenade launcher, night sights, etc...it would fall out of C&R immunity and become illegal. Adding a detachable magazine even just a 10 round detachable has been declared by the ATF in various letters as a violation of 922. How can some one have an illegal sks? Add: detachable magazine, folding stock, pistol grip, or bayonet.
If you want to modify an import and want to stay legal the best thing to do is to reduce the total listed foreign parts to 10 or less.
This is how the romanian AK-47s are compliant.
Importable shotguns can only have 5 round tube magazines.
Adding extensions to make it more than 5 rounds violates 922 and welcomes you to an all expenses paid trip to CLUB FED (prison)
Benelli no longer offers magazine extensions because the ATF realized that most people were not going to reduce the benelli total foreign parts to 10 in order to add the magazine extension.
922 (r)
The Imported Parts Law(1990)
178.39 otherwise known as 922(r) 10 Foreign parts law on semiauto Rifles & Shotguns
http://www.atf.treas.gov/regulations/27cfr178.html
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
(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) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates