Parts kit
A parts kit is a collection of weapon parts that, according to the Bureau of Alcohol, Tobacco, Firearms, and Explosives, "is designed to or may be readily be assembled, completed, converted, or restored to expel a projectile by the action of an explosive." As an example, the kit may not include a receiver or include an incomplete receiver. Under current U.S. law, kits that include finished receivers must be serialized and their buyers must receive a background check, but kits that include "unfinished" receivers are totally unregulated for purchase.
Receivers
US parts kit regulation is distinct from that of other countries, where a firearm's pressure bearing parts such as bolts, barrels, and gas pistons are the commonly regulated components. In the United States a serialized receiver can be purchased or manufactured from a state of incompleteness to create a firearm.The National Firearms Act restricts the possession of automatic firearms, so most parts kits end up used with a semi-automatic receiver. In addition, under US gun law, a receiver that is legally a machine gun cannot legally become semi-automatic. There is no federal restriction on the purchase and import of machine gun parts kits, however.
Parts kits are available for many firearms including the AR-15 and AKM variants.