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In firearms terminology, the firearm receiver or firearm frame is the part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel. The receiver is often made of forged, machined, or stamped steel or aluminium; in addition to these traditional materials, modern science and engineering have introduced polymers and sintered metal powders to receiver construction.
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In US law
For the purposes of United States law, the receiver or frame is legally the firearm, and as such it is the controlled part. The definition of which assembly is the legal receiver varies from firearm to firearm, under US law. For an AR-15 rifle, the lower assembly is considered the legal receiver. In a rifle such as the FN-FAL, it is the upper receiver that is serialized. For a FAL rifle, the upper assembly is considered the legal receiver.
Generally, the law requires licensed manufacturers and importers to mark the designated receiver with a serial number, the manufacturer or importer, the model and caliber. In addition, makers of receivers are restricted by International Traffic in Arms Regulations. Thus, in the case of a firearm that has multiple receivers (such as the AR-15, which has an upper and lower receiver), the legally controlled part is the one that is serialized (ATF has designated the lower in the AR-15's case).
Unfinished receivers
"Unfinished receivers", also called "80 percent receivers" or "blanks", are partially completed receivers with no serial numbers. Purchasers must perform their own finishing work in order to make the receiver usable. The finishing of receivers for sale or distribution by unlicensed persons is against US law. Because an unfinished 80% receiver is not a firearm, purchasers do not need to pass a background check. The resulting firearm is sometimes called a "ghost gun". An AR-15 variant made from an 80% receiver was used in the 2013 Santa Monica shooting.
Notable receivers
Two 3D printed polymer lower receivers for the AR-15 have been released: the AR Lower V5 and the Charon.