A U.S. parts kit is a is a collection of U.S. manufactured AK parts, usually containing seven U.S. manufactured parts.  Examples: butt stock, pistol grip, gas tube cover, hand guards, and a three piece trigger set, all made in the USA.  For a complete parts description click here.  By installing a U.S. Part kit on your existing "imported sporting rifle" you reduce the total imported parts count to less than ten, causing the gun not to be subject to sec. 922(r) as stated below.  In English this means that you can now have two bad features on your imported, now U.S. made AK, one being the pistol grip and the detachable magazine plate.

Please note that you still CANNOT install a threaded muzzle brake (unless it is tack welded) ,a bayonet lug or flash hider.  If you decide to install a muzzle brake you must replace another imported part with a U.S. manufactured part.  You may also use a U.S. muzzle brake in place of a U.S. part.

Official Statement
The Law states:
Section 922(r) Ruling applies. Please check local, state, and federal laws. Contact B.A.T.F for clarification concerning modifications. The B.A.T.F has ruled it is not lawful to modify any MAK-90, or Variant except by class II license holders in limited situations. Section 922(r) Rulings apply, which does not prohibit the importation, sale, or possession of parts or components which may be used to assemble a semi-automatic rifle or shotgun in violation of the statute-while stating that it shall be unlawful for any one to assemble from imported parts any semi-automatic rifle which is identical to any rifle prohibited from importation as not being particularly suitable or readily adaptable to sporting purposes, with certain exceptions. A rifle assembled or reconfigured to have 10 or less of the imported parts listed in paragraph 178.39(c) is not subject to the provisions of sec. 922(r) which prohibits the assembly  from imported parts a rifle which is ban from importation.

Types of U.S. Parts Kits

The basic kit comes with the following U.S. parts: hammer, trigger, disconnector, butt stock, pistol grip and upper/lower forearm. Unlike the magazine kit, the basic kit allows for the use of any magazine or drum.  The magazine kit only allows the use of U.S. made magazines and drums.  Most kits will come with complete instructions on how to remove the old parts and install the new U.S. made ones.  All kits will come with a B.A.T.F. letters of approval just incase you are questioned on the legality of your rifle.  Some U.S. Kit manufacturers provide a double arm trigger instead of a single arm trigger.  However most manufacturers will replace the double arm trigger with a single arm trigger if you request it.  A double armed trigger is a trigger with two arms that releases the hammer instead of one.  The benefits of installing a double arm trigger are better trigger pull and elimination of trigger slap.  However if you mess up, you have just ruined your receiver.  

The magazine kit comes with the following U.S. parts: disconnector, butt stock, pistol grip, upper/lower forearm, three magazine followers and three US magazine floor plates. Instead of replacing your imported trigger group, the kit allows you to replace magazine parts so that you may comply with U.S. law. This kit dose NOT allow the use of any non U.S. manufactured magazines and will  be in violation of U.S. law should you choose to ignore it.  While this kit is easier to install it severely limits your magazine capacity and options.  AK-47.net would not suggest purchasing this kit due to the legal problems you may encounter, and the ridiculous magazine restrictions.