http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf
Found on another forum...
From the link... signed by Melson...
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when parts in a kit that were originally designed to be configured as both a pistol and a rifle are assembled or re-assembled in a configuration not regulated under the NFA (e.g., as a pistol, or a rifle with a barrel of 16 inches or more in length).
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a barrel of 16 inches or more in length, and the parts are later unassembled in a configuration not regulated under the NFA (e.g., as a pistol).
Found on another forum...
From the link... signed by Melson...
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when parts in a kit that were originally designed to be configured as both a pistol and a rifle are assembled or re-assembled in a configuration not regulated under the NFA (e.g., as a pistol, or a rifle with a barrel of 16 inches or more in length).
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a barrel of 16 inches or more in length, and the parts are later unassembled in a configuration not regulated under the NFA (e.g., as a pistol).