Firearms. A weapon, and all components and parts therefor, not over .50 caliber which will or is designed to or may be readily converted to expel a projectile by the action of an explosive, but shall not include BB and pellet guns, and muzzle loading (black powder) firearms (including any firearm with a matchlock, flintlock, percussion cap, or similar type of ignition system) or firearms covered by Category I established to have been manufactured in or before 1898.
27 CFR § 447.11
When used in this part and in forms prescribed under this part, where not otherwise distinctly expressed or manifestly incompatible with the intent thereof, terms shall have the meanings ascribed in this section. Words in the plural form shall include the singular, and vice versa, and words imparting the masculine gender shall include the feminine. The terms includes and including do not exclude other things not enumerated which are in the same general class or are otherwise within the scope thereof.