To shorten our barrels, we used a hacksaw instead of a parting tool mounted on a lathe. We had lead jaw pads on our vise to prevent the polished barrel from being scratched and then went at it with a hacksaw.
The barrel was then wrapped in masking tape to prevent it from being scratched when placed back into a four jaw chuck. Shim made from a soda can protected the barrel where the jaws clamped down on it. Then a spud the diameter of the bore was inserted into the bore and a dial indicator placed against the spud to ensure that the barrel was concentrically mounted in the four jaw chuck. Remember, all that tape and the aluminum shims can cause the barrel to be mounted some what offset. Hence the use of a four jaw over a three jaw chuck. With the four jaw you can adjust the jaws until the workpiece is perfectly concentric.
Next we ground a crowning bit which was placed against the muzzle to crown it. The bit was placed 90 degrees to the muzzle and was concave shaped. It was fed straight in when we moved the saddle forward (toward the headstock). That gave us the decorative crown. For the actual crown, we used the Brownell hand turned crowning tool that had a pilot that dropped into the bore to keep the tool straight. Gentle pressure and the crown has to be chatter free to be acceptable.
Vigilantibus et non dormientibus jura subveniunt. Molon Labe!