Here is an excerpt from BATFE's decision and a link to the website.
"Therefore, so long as a parts kit or collection of parts is not used to make a firearm regulated under the NFA (e.g., a short-barreled rifle or "any other weapon" as defined by 26 U.S.C. 5845(e)), no NFA firearm is made when the same parts are assembled or re- assembled in a configuration not regulated under the NFA (e.g., a pistol, or a rifle with a barrel of 16 inches or more in length). Merely assembling and disassembling such a rifle does not result in the making of a new weapon; rather, it is the same rifle in a knockdown condition (i.e., complete as to all component parts). Likewise, because it is the same weapon when reconfigured as a pistol, no "weapon made from a rifle" subject to the NFA has been made.
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."