receiver

In firearms terminology and law, 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. Some firearm designs, such as the AR-15 platform, feature receivers that have 2 separate sub-assemblies called the upper receiver which houses the barrel/trunnion, bolt components etc and the lower receiver (trigger mechanism housing in some cases) that holds the fire control group, pistol grip, selector, stock etc.
The receiver is often made of forged, machined, or stamped steel or aluminium. Apart from these traditional materials, modern techniques have introduced polymer and sintered metal powder receivers to the market.

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