Quoted:
False. The Thompson/Center case established that as long as you have a LEGAL combination for the extra parts (i.e., 10.5" upper for the M16), and you don't have those parts illegally assembled (putting that 10.5" upper on the SP1), then you're fine.
View Quote
Pretty much, yes. United States v. Thompson/Center Arms Co., 504 U.S. 505 states in part:
"We also think that a firearm is "made" on facts one step removed from the paradigm of the aggregated parts that can be used for nothing except assembling a firearm. Two courts to our knowledge have dealt in some way with claims that when a gun other than a firearm was placed together with a further part or parts that would have had no use in association with the gun except to convert it into a firearm, a firearm was produced. See United States v. Kokin, 365 F.2d 595, 596 (CA3) (carbine together with all parts necessary to convert it into a machinegun is a machinegun), cert. denied, 385 U.S. 987, 17 L. Ed. 2d 448, 87 S. Ct. 597 (1966); see also United States v. Zeidman, 444 F.2d 1051, 1053 (CA7 1971) (pistol and attachable shoulder stock found "in different drawers of the same dresser" constitute a short-barreled rifle). Here it is true, of course, that some of the parts could be used without ever assembling a firearm, but the likelihood of that is belied by the utter uselessness of placing the converting parts with the others except for just such a conversion. Where the evidence in a given case supports a finding of such uselessness, the case falls within the fair intendment of "otherwise producing a firearm." See 26 U. S. C. § 5845(i)."
Of course, if the extra parts are NOT utterly useless except to make an illegal SBR (because you have a registered SBR or MG) then possession of additional <16" uppers should be fine.
It seems, therefore, that ATF's "constructive possession" theory fails if the unassembled part has a legal use by the owner, even if it also has an illegal use.