I do have one question in regards to making a SBR or really any NFA item. You mentioned that just having say an AR receiver and the short barrelled upper receiver could be considered constructive possession. So is it possible to just have the receiver, register that as the SBR, then purchase the upper afterwards? Or is it required on the form to report the length of the barrel or something. I am looking at possibly going that route with an SBR starting with an AR receiver.
You should obtain your short barreled upper only after you have received an approved Form 1. You will have to mark on the Form 1 both the barrel length and the overall length of the item being registered. You can guesstimate the overall length. With your AR assembled with a 16" barrel, measure the overall length, then subtract the difference in barrel length for you short barrel. Also, ATF considers the overall length of the gun with a folding or collapsable stock with the stock in its longest position.
--EasyD