I referred to the US v. Thompson/Center case above, but will elaborate some more.
The T/C case dealt with packaging a Contender pistol with a long barrel and rifle stock so that the pistol purchaser may convert the pistol into a rifle and back (Legal; converting an original rifle into a pistol is not legal).
However, the parts could be combined in a way to create a contender rifle with the pistol barrel (an SBR).
The Supreme Court, in a very mixed decision, ruled that the NFA statute regarding "making" a short barreled rifle (NFA "firearm") from parts and a non-NFA gun was ambiguous. Therefore, the rule of lenity required that the possession of parts that have a legitimate purpose to create non-NFA guns (i.e. a legal psitol and a legal rifle) do not in themselves constitute an NFA firearm.
"Here, however, we are not dealing with an aggregation of parts that can serve no useful purpose except the assembly *513 of a firearm, or with an aggregation having no ostensible utility except to convert a gun into such a weapon. There is, to be sure, one resemblance to the latter example in the sale of the Contender with the converter kit, for packaging the two has no apparent object except to convert the pistol into something else at some point. But the resemblance ends with the fact that the unregulated Contender pistol can be converted not only into a short-barreled rifle, which is a regulated firearm, but also into a long-barreled rifle, which is not. The packaging of pistol and kit has an obvious utility for those who want both a pistol and a regular rifle, and the question is whether the mere possibility of their use to assemble a regulated firearm is enough to place their combined packaging within the scope of "making" one."
504 U.S. 505 at 513-514.
"After applying the ordinary rules of statutory construction, then, we are left with an ambiguous statute. ... Accordingly, we conclude that the Contender pistol and carbine kit when packaged together by Thompson/Center have not been "made" into a short-barreled rifle for purposes of the NFA."
504 U.S. 505 at 517-518.
Definition of the rule of lenity: "Under the common law rule of lenity, courts must strictly construe penal statutes in order to avoid a violation of the due process rights of the accused. Thus, in criminal cases where two reasonable interpretations of a penal statute exist, one inculpating and the other exculpating a defendant, a court must employ the less harsh reading."
Therefore, it seems that the Supreme Court's position of owning a non-NFA rifle that is compatible with the short-barreled uppers for a registered NFA firearm (an SBR) is based on the fact that you have a legitimate purpose for owning the short-barreled uppers. Kent did not have what the 11th Circuit Court felt was a legitimate purpose for owning the complete and usable short-barreled upper in his small apartment, except for making an unregistered SBR.
Caveat: I do not know if the NFA statutes have been rewritten or modified from the date of the Supreme Court's decision that would remove the ambiguity they cite.
Cheers, Otto
Edited to add: This really needs a spell-checker feature.