Okay, this question isn't as easy as the title makes it out to be; what I have is truly a rare situation, where I have an SR-25 patterned lower receiver that has been registered & transferred as a 'Shotgun' chambered in 12ga (it has such a corresponding upper).
Now, I want to attach my rifled .308 upper to this lower. Mechanically it works of course, but legally what is the barrel length restriction? I'm going back down to a sub 0.5" bore size rifled barrel, so whether it's now a DD isn't to be called into question, but is my legal barrel length minimum still 18", or can I put a 16" on it?