Just put a nice, heaviy zip tie through the mounting pins of the upper receiver to clearly demonstrate that you are not attempting to use it without written approval, and you should have no difficultoes. I think where people run into unnecessary risk is when they try to play the "it's a pistol" game on a standard receiver, or when they have SBR uppers and regular lowers sitting around, but no regular uppers (16") on hand. ATF guys aren't stupid; if it looks like you're going out of your way to insure compliance, they aren't likely to give you any trouble, if indeed you ever see one. It's the guys that play the margins and try to get away with borderline calls that usually end up getting unwanted attention.

AC