i don't know for sure, but while at a gunshow, someone said it is legal to own a pistol-upper without a permit. it is only a gun-part at that point. when the upper is ASSEMBLED onto a lower,  then one must have an SBR lic. or pistol permit. i doubted him, but then figured that companies like ASA, M&A Sales and Oly arms will ship a pistol upper to anyone, anywhere without proof of a permit, FFL or anything else - a COMPLETE firearm is a dif. story. hope this helps!

yeah just when selling the uppers have a sign next to them saying ALL NFA rules and 1994 Crime bill Rules Apply!! just to cover your ass

As long as it's not installed on a lower it's no problem at all.  I see short AR barrels and even complete "liberated" M4 uppers for sale all the time, even here in the Occupied California Republic.

Much like buying a new slide for your 1911 or glock is perfectly doable without any paperwork other than your credit card info.

The only regulated item is the part with the recorded serial number on it.