The ATF made the rule awhile ago under ATF 2011-4
The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) has received requests from individuals to classify pistols that are reconfigured into rifles, for personal use, through the addition of barrels, stocks, and other parts and then returned to a pistol configurationbyremovalofthosecomponents. Specifically,ATFhasbeenaskedto determine whether such a pistol, once returned to a pistol configuration from a rifle, becomes a “weapon made from a rifle” as defined under the National Firearms Act (NFA).
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.