garandman: To clarify, an upper is not a firearm, so it's not subject to regulation.
You cannot legally possess a short barrel for your AR-15, though, as ATF classifies that possession as clear evidence of an attempt to build an unregistered short barrel rifle.
If you already own an AR-15, the legal way to make an SBR is to fill out a Form 1, with $200 tax payment, CLEO signature, pics and prints, and wait for it to come back approved with your tax stamp. You can then legally take delivery of a short-barrel upper, and mount it on your lower (after marking it as required).
If your lower is pre-ban, it can be a pre-ban configuration upper, otherwise, it has to be in a post-ban legal configuration (which, for a semi-auto with a detachable magazine and pistol grip, means no bayo lug, threaded barrel or flash hider.)