Short-barreled shotgun.

Short-barreled shotgun. A shotgun having one or more barrels less than 18 inches in length, and any weapon made from a shotgun, whether by alteration, modification, or otherwise, if such weapon as modified has an overall length of less than 26 inches.

Source

27 CFR § 478.11

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.