The Gun Control Act of 1968 dictates that a rifle must have a barrel length of at least 16" and an overall length of at least 26". If either condition is not met, it is considered a short barreled rifle (SBR) and is subject to registration.
It is legal for you to have the 11.5 barrel as long as you permanently mount a muzzle brake/flash hider brings the length up to the minimum 16"
Straight from the ATF wbsite:
(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 the explosive in a fixed metallic cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger.
(8) 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.