Definition of Frame or Receiver and Identification of Firearms
A Rule by the Alcohol, Tobacco, Firearms, and Explosives Bureau on 04/26/2022
Bureau of Alcohol, Tobacco, Firearms, and Explosives; Department of Justice.
I. Executive Summary
A. Summary of the Regulatory Action
There are no statutory definitions for the terms frame or receiver in the Gun Control Act of 1968 (GCA) or the National Firearms Act of 1934 (NFA). To implement these statutes, the terms firearm frame or receiver and frame or receiver were defined in regulations to mean [t]hat part of a firearm which provides housing for the hammer, bolt or breechblock, and firing mechanism, and which is usually threaded at its forward portion to receive the barrel. 27 CFR 478.11 (implementing GCA, Title I); 27 CFR 479.11 
(implementing GCA, Title II). These definitions were meant to provide direction as to which portion of a weapon is the frame or receiver for purposes of licensing, serialization, and recordkeeping, thereby ensuring that a component necessary for the functioning of the weapon could be traced if later involved in a crime.
However, a restrictive application of these definitions would not describe the frame or receiver of most firearms currently in circulation in the United States. Most modern weapon designs, including semiautomatic rifles and pistols with detachable magazines, have a split or multi-piece receiver where the relevant fire control components are housed by more than one part of the weapon (e.g., the upper receiver and lower receiver of an AR-15 rifle), or incorporate a striker to fire the weapon, rather than a hammer.
In the past few years, some courts have treated the regulatory definition of firearm frame or receiver as inflexible when applied to the lower portion of the AR-15-type rifle, one of the most popular firearms in the United States. If broadly followed, that result could mean that as many as 90 percent of all firearms (i.e., with split frames or receivers, or striker-fired) in the United States would not have any frame or receiver subject to regulation. Furthermore, technological advances have also made it easier for companies to sell firearm parts kits, standalone frame or receiver parts, and easy-to-complete frames or receivers to unlicensed persons, without maintaining any records or conducting a background check. These parts kits, standalone frame or receiver parts, or partially complete frames or receivers enable individuals to make firearms quickly and easily. Such privately made firearms (PMFs), when made for personal use, are not required by the GCA to have a serial number placed on the frame or receiver, making it difficult for law enforcement to determine where, by whom, or when they were manufactured, and to whom they were sold or otherwise transferred. Because of the difficulty with tracing illegally sold or distributed PMFs, those firearms are also commonly referred to as ghost guns.