Pistols.

Pistols. Small projectile firearms which have a short one-hand stock or butt at an angle to the line of bore and a short barrel or barrels, and which are designed, made, and intended to be aimed and fired from one hand. The term does not include gadget devices, guns altered or converted to resemble pistols, or small portable guns erroneously referred to as pistols, as, for example, Nazi belt buckle pistols, glove pistols, or one-hand stock guns firing fixed shotgun or fixed rifle ammunition.

Source

27 CFR § 53.11

Scoping language

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 importing 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.