The ATF has defined the overall length of a firearm as follows.

The overall length of a firearm is the distance between the muzzle of the barrel and the rearmost portion of the weapon measured on a line parallel to the axis of the bore. ¹

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. 7.