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.

https://www.govinfo.gov/content/pkg/USCODE-2012-title18/html...

A rifled bore, for those not familiar with firearms is grooved to impart spin stabilization to the projectile. Pistols also have rifled bores, but are intended to be held in the hand when fired.