ATF. The Bureau of Alcohol, Tobacco, Firearms and Explosives, Department of Justice.
27 CFR § 555.11
When used in this part, terms are defined as follows in this section. Words in the plural form include the singular, and vice versa, and words indicating the masculine gender include the feminine. The terms includes and including do not exclude other things not named which are in the same general class or are otherwise within the scope of the term defined.