Parts kit

A parts kit is a kit of firearm parts without the receiver. This is due to US gun laws that consider the receiver the "gun" part of the firearm and the part that is regulated. This is different from other countries where pressure bearing parts such as bolts, barrels, and gas pistons are the regulated parts. A receiver can be purchased or manufactured to complete the firearm.
The National Firearms Act (NFA) 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 (minus the barrel), however.
Parts kits are available for many firearms including the AR-15 and AKM variants. 