That example - it's not an SBR upper, it's just an upper that's not wearing a 16" barrel. Just a shorter barrel. Is that an SBR upper, or is that a pistol upper? You can now have one lower, a +16" upper, and a less-than-16" upper, all together. ATF Ruling 2011-4 stated that. http://www.atf.gov/regulations-rulings/rulings/atf-rulings/atf-ruling-2011-4.pdf http://308ar.com/forum/firearm-industry-news-and-gossip/atf-ruling-2011-4-concerning-ar-pistols-and-rifles/ http://308ar.com/forum/firearm-industry-news-and-gossip/atf-ruling-2011-4-concerning-ar-pistols-and-rifles/ As soon as they permitted the individual to build a pistol (first action), and allowed you to turn in into a rifle (that part is not new), and then revert that firearm back to a pistol (this is new). As long as it was a pistol build first, it can go to rifle configuration, and back to a pistol again. Back and forth. A person can accomplish that with one built lower, two uppers, but with two receiver extension options. Buttstock and pistol config. Perfectly legal. What would they say if you have the lower assembled with the buttstock, and both uppers loose in the safe? Don't know. Legally, can you do that? Yes, per their ruling. Better have that pistol receiver extension close by, too, though. A short upper is not an SBR. It's just an upper with a barrel less than 16". Putting it onto a rifle-config lower, without a stamp - that's the crime. Not assembling it onto a rifle-config lower, is the key. Only put it on a pistol lower if you don't have the SBR tax stamp. With my pistol build, I locked it up with all the other rifles. Not worried at all about storing it that way. It stayed together as a pistol. I even had a spare upper for it. I kept them together - complete pistol with spare pistol upper. Those weren't SBRs, not in the least.