According to the feds, the gun mentioned by the OP would appear to be a rifle:

18 U.S. Code § 921 - Definitions

(a) As used in this chapter—

Paragraph a(5) The term “shotgun” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger.

Paragraph a(7) The term “rifle” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of an explosive to fire only a single projectile through a rifled bore for each single pull of the trigger.

If you want to stay out of trouble with a given state in a rifle vs. shotgun issue, you'd want to read that state's definitions.