In firearms terminology and law, the firearm frame or receiver is the part of a

firearm A firearm is any type of gun that uses an explosive charge and is designed to be readily carried and operated by an individual. The term is legally defined further in different countries (see legal definitions). The first firearms originate ...

which integrates other components by providing

housing Housing refers to a property containing one or more Shelter (building), shelter as a living space. Housing spaces are inhabited either by individuals or a collective group of people. Housing is also referred to as a human need and right to ...

for internal action components such as the

hammer A hammer is a tool, most often a hand tool, consisting of a weighted "head" fixed to a long handle that is swung to deliver an impact to a small area of an object. This can be, for example, to drive nail (fastener), nails into wood, to sh ...

, bolt or breechblock, firing pin and extractor, and has threaded interfaces for externally attaching ("receiving") components such as the

barrel A barrel or cask is a hollow cylindrical container with a bulging center, longer than it is wide. They are traditionally made of wooden stave (wood), staves and bound by wooden or metal hoops. The word vat is often used for large containers ...

stock Stocks (also capital stock, or sometimes interchangeably, shares) consist of all the Share (finance), shares by which ownership of a corporation or company is divided. A single share of the stock means fractional ownership of the corporatio ...

, trigger mechanism and

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

/ 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

forge A forge is a type of hearth used for heating metals, or the workplace (smithy) where such a hearth is located. The forge is used by the smith to heat a piece of metal to a temperature at which it becomes easier to shape by forging, or to the ...

d, machined, or stamped

steel Steel is an alloy of iron and carbon that demonstrates improved mechanical properties compared to the pure form of iron. Due to steel's high Young's modulus, elastic modulus, Yield (engineering), yield strength, Fracture, fracture strength a ...

aluminium Aluminium (or aluminum in North American English) is a chemical element; it has chemical symbol, symbol Al and atomic number 13. It has a density lower than that of other common metals, about one-third that of steel. Aluminium has ...

. Apart from these traditional materials, modern techniques have introduced

polymer A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...

and sintered metal powder receivers to the market.

Mounting

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

In US law

For the purposes of

United States law The law of the United States comprises many levels of Codification (law), codified and uncodified forms of law, of which the supreme law is the nation's Constitution of the United States, Constitution, which prescribes the foundation of the ...

, 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 A serial number (SN) is a unique identifier used to ''uniquely'' identify an item, and is usually assigned incrementally or sequentially. Despite being called serial "numbers", they do not need to be strictly numerical and may contain letters ...

, the manufacturer or importer, the model and caliber. Makers of receivers are restricted by International Traffic in Arms Regulations. Thus, in the case of a firearm that has multiple receiver parts, such as the AR-15, which has an upper and a lower receiver, the legally controlled part is the one that is serialized. For the AR-15 rifle, the lower receiver assembly is legally considered the actual receiver, although it is functionally a chassis that also houses the separate trigger group. In the FN-FAL rifle, it is the upper assembly that is serialized and legally considered the receiver. 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.

Unfinished receivers

"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. Because an unfinished 80% receiver is not a firearm, purchasers do not need to pass a background check. The resulting firearm is known as a " ghost gun". During the Biden Administration, the ATF 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. In the case VanDerStok 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. However, on August 8, 2023, the Texas court's nationwide vacatur was temporarily placed on hold by the

Supreme Court of the United States The Supreme Court of the United States (SCOTUS) is the highest court in the federal judiciary of the United States. It has ultimate appellate jurisdiction over all Federal tribunals in the United States, U.S. federal court cases, and over Stat ...

, leaving the new ATF regulations on unfinished receivers in place.

3D printed receivers

, several designs and at least two designs for 3D printable polymer lower unfinished receivers for the AR-15 have been released: the AR Lower V5 and the Charon. 3D printed designs may also be used to produce privately made firearms.

In Canadian law

The receiver or frame is legally the firearm, and as such it is the part that requires a manufacturer's

and valid Possession and Acquisition Licence to acquire and own. In the case of a handgun frame or revolver frame, it is the part that requires a Restricted-class PAL (RPAL), and registration.

References

{{Firearm accessories mounting standards Firearm components