When you fill out your form 1 you put the serial of the frame you want to register and what cartridge/barrel length the SBR will be built as. so let's say you wanted a 12" 44 magnum. You would put a 12" 44 magnum barrel, with the frame serial and OAL with the stock on the form then that would be the registered setup.
You can own as many barrels as you want and your good to go. They recommend that you put it back into the safe in the "registered configuration" not sure how critical that is as you could just jam it on if you were ever questioned.
I have a few AR SBR's and will be doing my Coupe de grace bolt action when I get it so I can use it as a specialty pistol, SBR, or rifle as I please.
The only thing that sucks is if I want to travel out of the state you have to fill out a form and get it approved....aside from that and then $200 tax/delayed infringement they rock!