Infirearms terminology and law, thefirearm frame orreceiver is the part of afirearm which integrates other components by providinghousing for internalaction components such as thehammer,bolt orbreechblock,firing pin andextractor, and hasthreaded interfaces for externally attaching ("receiving") components such as thebarrel,stock,trigger mechanism andiron/optical sights.^[1] Some firearm designs, such as theAR-15 platform, feature receivers that have 2 separate sub-assemblies called theupper receiver which houses the barrel/trunnion, bolt components etc and thelower receiver (Trigger Mechanism Housing in some cases) that holds the fire control group, pistol grip, selector, stock etc.^[2]

The receiver is often made offorged, machined, or stampedsteel oraluminium. Apart from these traditional materials, modern techniques have introducedpolymer andsintered metal powder receivers to the market.^[3]

A barrel may typically be affixed to a firearm receiver using barrel and receiveraction threads or similar methods.

For the purposes ofUnited States law, the receiver or frame is legally the firearm,^[4] 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 aserial number, the manufacturer or importer, the model and caliber. Makers of receivers are restricted byInternational Traffic in Arms Regulations. Thus, in the case of a firearm that has multiple receiver parts, such as theAR-15, which has an upper and a lower receiver, the legally controlled part is the one that is serialized.^[5]

For the AR-15 rifle, the lower receiver assembly is legally considered the actual receiver,^[5] although it is functionally achassis that also houses the separatetrigger group. In theFN-FAL rifle, it is the upper assembly that is serialized and legally considered the receiver.^[6][7] This has led to prosecutors dropping charges against illegal manufacturing of AR-type firearms to avoid court precedents establishing that neither the upper nor the lower receiver individually contain all the components to be legally classified as a firearm.^[8]

"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.^[9] Because an unfinished 80% receiver is not a firearm, purchasers do not need to pass a background check.^[9] The resulting firearm is known as a "ghost gun".^[10]

During theBiden Administration, theATF imposed regulations on the sale and marketing of unfinished receivers and kits containing them by revising the legal definition of receivers to include "a partially complete, disassembled, or nonfunctional frame or receiver, including a frame or receiver parts kit, that is designed to or may readily be completed, assembled, restored, or otherwise converted to function as a frame or receiver". (The term "receiver" was also redefined as referring to non-handgun firearms, while "frame" was redefined as referring to handguns exclusively.) The new definitions went into effect on August 24, 2022.^[11] In the caseVanDerStok v. Garland, filed on June 30, 2023, a federal court in Texas ruled that the new ATF regulations exceeded the agency's statutory authority, and struck them down.^[12] However, on August 8, 2023, the Texas court's nationwidevacatur was temporarily placed on hold by theSupreme Court of the United States, leaving the new ATF regulations on unfinished receivers in place.^[13]

As of 2024^[update], several designs and at least two designs for3D printable polymer lower unfinished receivers for the AR-15 have been released: theAR Lower V5 and theCharon. 3D printed designs may also be used to produceprivately made firearms.^[14]

The receiver or frame is legally the firearm, and as such it is the part that requires a manufacturer'sserial number and validPossession and Acquisition Licence to acquire and own. In the case of ahandgun frame or revolver frame, it is the part that requires a Restricted-classPAL (RPAL), andregistration.