To clear up any confusion on "constructive possession" and the belief that there is no such thing. There is established case law.
In United States v. Thompson/Center Arms Company, 504 U.S. 505 (1992), the United States Supreme Court examined whether a short-barreled rifle was “made” under the NFA when a carbine-conversion kit consisting of a single-shot “Contender” pistol was designed so that its handle and barrel could be removed from its receiver, and was packaged with a 21-inch barrel, a rifle stock, and a wooden fore-end. The Court held that, where aggregated parts could convert a pistol into either a regulated short-barreled rifle, or an unregulated rifle with a barrel of 16 inches or more in length, the NFA was ambiguous and applied the “rule of lenity” (i.e., ambiguities in criminal statutes should be resolved in favor of the defendant) so that the pistol and carbine kit, when packaged together, were not considered a “short-barreled rifle” for purposes of the NFA.
However, the Court also explained that an NFA firearm is made if aggregated parts are in close proximity such that they: (a) serve no useful purpose other than to make an NFA firearm (e.g., a receiver, an attachable shoulder stock, and a short barrel); or
(b) convert a complete weapon into an NFA firearm (e.g., a pistol and attachable shoulder stock, or a long-barreled rifle and attachable short barrel). Id. at 511-13.
ATF SBR Ruling 2011-4