It is just as easy to register the shotgun as a SBS (short-barreled shotgun) as a SBR, so I don't see any advantage to the latter. Nothing precludes hanging the SBS under the barrel of the rifle...at least not on a federal level. The rifle becomes the stock for the SBS, but does not change the status of the rifle. Local ordinances might have an impact. The transfer tax on the SBS/SBR is $200...only $5 on the AOW. But it costs $200 to MAKE the AOW, so most folks have a Class II manufacturer "make" it and then transfer it to them.