Interesting. Was not aware of that letter. But it doesn't address the case of having another allowed configuration for that short barrel, i.e. a pistol lower. So does this mean that I can't take my AR pistol and my 16" AR (registered lower) on the same road trip out of state without paperwork? I was going under the concept of "constructive possession" where there must be no other use for the components other than the regulated use. Otherwise, this would create the problem where it would be a violation to own both an AR pistol and an AR rifle, since the parts are interchangeable. I don't think it works that way. If your interpretation is correct it changes the rules from "may only be assembled as" to simply "may be assembled as". ETA: In my original reply I was assuming, but did not state, that both the pistol lower AND the rifle (SBR) lower would be present when traveling. This may be the source of confusion. Both must be present so that there is always a "legal" configuration for the parts. There must also be a 16" upper present so that there is a reason for the rifle lower other than to make an SBR.