The ATF defines a rifle as a firearm designed to be fired from the shoulder and designed to use the energy of an explosive in a fixed cartridge to fire only a single projectile through a rifled barrel for each single pull of the trigger. 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. 6.