upper receiver

In firearms terminology, the firearm frame or receiver is the part of a firearm which integrates other components by providing housing for internal action components such as the hammer, bolt or breechblock, firing pin and extractor, and has threaded interfaces for externally attaching ("receiving") components such as the barrel, stock, trigger mechanism and iron/optical sights. 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.

View More On Wikipedia.org