As defined by the U.S. Government Publishing Office (GPO), the Code of Federal Regulations (CFR) is the codification of the general and permanent rules published in the Federal Register by the departments and agencies of the Federal Government. ¹

As the ATF so succinctly puts it, CFR means the Code of Federal Regulations in which Federal firearms regulations are published. ²

1. Source: https://www.gpo.gov/help/about_code_of_federal_regulations.htm ⇒
2. National Firearms Act Handbook. ⇒ U.S. Dept. of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives, Office of Enforcement Programs and Services, 2009. p. 2.