What is the minimum legal length for a rifle?

Answer 1

The following applies to Federal Laws of the USA. 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. *

The term "short-barreled rifle" means a rifle having one or more barrels less than sixteen inches in length and any weapon made from a rifle (whether by alteration, modification, or otherwise) if such weapon, as modified, has an overall length of less than twenty-six inches. *

So, in order to remain compliant with federal code without having to register and pay a NFA tax on a rifle, the barrel of a rifle, as measured from the face of the closed bolt to the muzzle must be at least 16", and if you are shortening a rifle, the rifle must be at least 26" in length when it is ready to fire.

* http://www.law.cornell.edu/uscode/18/usc_sec_18_00000921----000-.html