Dave,
A tubing cutter tends to swage the tube down to a smaller diameter during the cutting process. Consequently, if it were my job to do, I would carefully mark the barrel around its entire circumference where I wanted the cut, slide a bore diameter dowel down the bore to where the cut is to be, clamp the portion to be removed in the vice and cut it off with a fine tooth hack saw. The dowel inside the bore will tend to minimize burr formation but you will still have a burr at the cut. After cutting you will have to check the cut for squareness with the axix of the bore and carefully file the muzzle end square. (If the final muzzle is not square the gun will shoot towards the short side). Carefully ream the cutting and filing burr out with a deburring tool or a ball file. As Darryl mentioned, most muzzles are crowned a bit to facilitate loading. A ball file is the ideal tool to use if you don't own a lathe (I don't). Usually after crowning a barrel I will take some 600 grit paper and fold it into a cone, shove a ball down inside the cone and use the ball to polish the crown until it is glassy smooth. This really aids in the loading.
Best Regards,
John Cholin