I have a question related to another thread that I was reading tonight. If you bought a 410 AR upper, cut the barrel down to 12", would you have to buy a stamp for a SBS, since it is a shotgun, or could you simply put it on your AR lower that is registered as a SBR? I would think that you could put it on your existing lower, since the lower is the part that is registered. Theoretically, you could use 9mm shotshell in a 9mm upper. That and you are registering the gun, not the ammo shot out of it. However, the case could be made that a shotgun is defined by the type of ammo it was designed for, like a 12 gauge shotgun with a 12" slug barrel. Would that be a SBS or a SBR?