<P> In firearms terminology, the firearm receiver or firearm frame is the part of a firearm which provides housing for internal components such as the hammer, bolt or breechblock, action and firing mechanism, and is usually threaded at its forward portion to "receive" the barrel and has screw holes on the bottom and / or rear to receive the stock and grip . 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 . </P> <P> 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 . 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 . For the AR - 15 rifle, the lower assembly is considered the legal receiver; while in the FN - FAL rifle, it is the upper assembly that is serialized and considered the legal receiver . </P>

What is the upper and lower of a gun