Contraband firearm. A firearm with respect to which there has been committed a violation of the National Firearms Act (26 U.S.C., Chapter 53) or any regulation issued thereunder.
27 CFR § 72.11
As used in this part, unless the context otherwise requires, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words importing the masculine gender shall include the feminine. The terms includes and including do not exclude things not enumerated which are in the same general class.