I thought about this alot. I think if you get the class three for SBR, you can fit any length barrel on your frame with a shoulder stock. I think one of the requirements is to stamp the caliber, name and city/state on the gun. What you can do is stamp every caliber you can think of using on the frame, or you can stamp it on a barrel, but you will have to keep that barrel with the frame at all times. I think it would be really cool to be able to use all the pistol barrels out there with a stock. Let us know if you do it and post some pics.