You might want to read this from BATFE:
Nonetheless, if a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length is assembled or otherwise produced from a weapon originally assembled or produced only as a rifle, such a weapon is a “weapon made from a rifle” as defined by 26 U.S.C. 5845(a)(4). Such a weapon would not be a “pistol” because the weapon was not originally designed, made, and intended to fire a projectile by one hand.
https://www.atf.gov/file/55526/download
And, focus on the part that says "...or otherwise produced from a weapon originally....produced only as a rifle..."
So, I'd think twice about assembling a pistol from receiver that is somehow designated as a carbine or rifle receiver. Whether or not it's a rifle is not limited to the installation of a certain length barrel from the factory.