The ATF defines a shotgun as a firearm designed to be fired from the shoulder and designed to use the energy of the explosive in a fixed shotgun shell to fire through a smooth bore either a number of projectiles or a single projectile for each pull of the trigger. A shotgun is not subject to the NFA if none of its barrel(s) is/are less than 18 inches in length and the overall length of the firearm is not less then 26 inches in length. ¹

See the information regarding barrel measurement for ATF accepted practices.

1. National Firearms Act Handbook. ⇒ U.S. Dept. of Justice, Bureau of Alcohol, Tobacco, Firearms and Explosives, Office of Enforcement Programs and Services, 2009. p. 5.