[dogtown tom]

zoomie

United States v. Thompson-Center Arms Co. ruled you had to show intent.

No, it didn't.
From your link:

Justice Souter wrote that “a set of parts that could be used to make nothing but a short-barreled rifle” would, if there is an “aggregation” of such, be a short-barreled rifle. The opinion states that “a combination of parts that could only be assembled into an NFA-regulated firearm” would be such a firearm.[1] Further, a non-NFA gun becomes an NFA firearm if “placed together with a further part or parts that would have no use in association with the gun except to convert it into a firearm.” As examples, the court mentions a carbine with a machinegun conversion kit,[2] and a pistol and attachable shoulder stock found in different drawers of the same dresser.

From Wikipedia:

The court ruled in Thompson Center Arms' favor in that the carbine conversion kit did not constitute a short-barreled rifle, primarily because the kit contained both the stock and the 16-inch barrel.

Justice Scalia also noted that there is a warning carved on the stock telling the user to not attach the stock to the receiver when the 10-inch barrel is attached to the receiver or vice versa.

This circumstance caused the court to apply the rule of lenity since the NFA carries criminal penalties with it. This meant that ambiguous statutes are interpreted against the government.

The TC Contender kit had a legal configuration for both pistol and rifle.