What is the "Frame" of a Firearm or Gun?

Revolver frame.
This photo shows most of the frame of a single action revolver. The frame is the part that looks more silver. Photo © Russ Chastain

The term "frame" or "receiver" is the metal portion of a firearm to which all the other components--the  trigger, hammer , barrel, etc.--are attached in such a manner that they work together to achieve proper functioning of the gun.

The frame is usually created from of forged, machined, or stamped steel or aluminium, but  some modern weapons may have frames made from polymers.   in addition to these traditional materials, modern science and engineering have introduced composite polymers  or composite metals. 

"Frame" or "receiver" are terms that may be used in reference to both handguns and long guns , although "receiver" usually applies to long guns such as rifles and shotguns, while "frame" is more often used in regards to handguns.

On most guns, the stamped serial number of the firearm is found on the frame. Manufacturers and importers are required by federal law to stamp the frames of all firearms with serial numbers for tracking purposes.  A firearm created from an unfinished frame without a serial number is known as a "ghost gun." It is illegal for individuals to sell or distribute unfinished frames without serial stamps, since a ghost gun created with such a frame is impossible to track in the event it is used in criminal activities.