Rifles are defined more by what they AREN'T than what they ARE. "Shoulderable", "rifled barrel", 16", and 26" are the points where the federal government classifies a firearm as something different, such as a shotgun or a handgun.
I'm 1000% serious, their rules are byzantine (and stupid).
https://cdn.shopify.com/s/files/1/0020/5370/1690/files/ar-pi...
This is pretty good too. Its not a rifle, its not a pistol, its "Any Other Weapon" apparently.
This is literally how arbitrary our regulations are. Criminals don't care about regulations, its proven over and over.
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.
According to the ATF: >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 the explosive in a fixed metallic cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.