Assuming that the firearm was originally a pistol, the resulting firearm, with an attached shoulder stock, is not an NFA firearm if it has a barrel of 16 inches or more in length.
Pursuant to ATF Ruling 2011-4, such rifle may later be unassembled and again configured as a pistol. Such configuration would not be considered a “weapon made from a rifle” as defined by 26 U.S.C. § 5845(a)(4).
[26 U.S.C. § 5845, 27 CFR § 479.11]