In an effort to convince you:
The horror of the statute is that they don't have to prove that you did ever assemble the short barreled upper to a non-NFA lower. They only have to prove that you have no other legitimate use for the upper but for making an SBR, in conformity with the US Supreme Court case of US v Thompson/ Center , 504 U.S. 505 (1988).
"While there is no direct evidence that Kent had assembled the rifle using the short-barreled upper receiver unit before, there was sufficient evidence, as outlined above, that Kent did not have the short-barreled upper receiver unit for parts, but for use with the lower receiver unit, which would constitute possession of a "firearm" required to be registered under § 5861(d) of the NFA."
US v Kent, 175 F.3d 870 at 878. (emphasis added)
Also, I really should clarify an earlier point:
"Short barrel = legal" is in itself, as just a barrel, ok, but a barreled upper receiver was specifically discussed in Kent:
"The Government introduced a videotaped demonstration which showed that it took only about thirty seconds to remove an AR-15 upper receiver unit from a lower receiver unit and install the short-barreled upper receiver unit in its place. The Government also introduced evidence that the upper receiver unit was a complete, intact unit that included not just a barrel, but also a flash suppressor, forward and rear sights, a scope with batteries to activate the light in the scope, a gas tube, a handguard assembly, and a sling ready to be attached to a lower receiver unit."
US v Kent, 175 F.3d 870 at 877.
From this, it can be argued that if you want to mount a muzzle device yourself on a new, sub-16 inch barrel, get just the barrel, make it permanently 16 inches or longer and then mount it to the upper receiver. Then, at no time is an SBR possible to be "made" in less than a minute.
Cheers, Otto