Get a copy of Brockways book on recreating muzzleloading shotguns. This is something that is covered extensively here. In short, you do not want to cut off the chamber since you will be running the tap directly into it for the threading operation. Also, most original plugs were very short, Brockway pretty much doubles the length of the plugs in his barrel for a couple reasons. Since chamber diameters varied considerably it is possible that you won't have full thread engagement, possibly only 50%. By doubling the length of the plug you can correct for this by bringing the thread engagement back closer to what it would be if you had full engagement on a shorter length plug.