[dogtown tom]

zoomie

And so can your AR as long as you have a rifle upper along with your pistol upper.

Careful. If "your AR" was originally a rifle, mere possession of a 7.5" upper means no lawful configuration as a Title I firearm. You have a lawful configuration as a rifle with a 16" barrel, but no lawful configuration with your 7.5" upper.

If you have other AR pistols or stripped lowers you can lawfully possess that 7.5" upper.

ONLY if the AR was originally assembled as a pistol can it be configured as a rifle and subsequently reconfigured as a pistol. As with the T/C kit.

If the AR was originally a rifle it cannot EVER be reconfigured as a pistol. It would be a "firearm made from a rifle" and subject to the NFA.

There are no such beasts as "rifle upper" or "pistol upper", just like there aren't "rifle lowers" or "pistol lowers".
Remember, you might have a 16" barrel, but less than 26"OAL.....and thats an NFA SBR.


Again,
You need to have a lawful configuration possible with the components in your possession.
Having a factory AR15 rifle with 16" bbl is fine....but being in possession of that 7.5" upper means you are in constructive possession of an SBR.

They'd have a case if you have one lower and only one upper, but that wasn't implied or stated in the question I responded to.

A case for what?
Possession of "...one lower and only one upper..." isn't a violation of any federal law.