Originally Posted by
Litpipe
The firearm as he originaly purchased it was not an sbr. He has done the paper, paid the tax and made it an sbr. He is now the manufacturer of that sbr, which is why he has to put his info on it.
The lower is registered as the sbr. Not the upper or barrel. The lower receiver is where the s/n, which is on file as an sbr, is located.
From now on, no matter what upper is attached, the lower is registered as an sbr. There is no registered configuration. If he engraves the lower he could have 20 uppers with 10" barrels and attach them at any time.