Try Slugging the bore. See what diameter you need. IF it has rifling, some lapping can help the even suface of the barrel and sharp edges of any pits. Have you thought of CAST bullets?? If you have a bore that is .316 or more, you should be able to use .318 jacketed softs points, or a close and slightly over groove diameter cast. The chamber neck may limit your bullet diamter. You could look at opening the neck up a bit an still use .303 British cases and the large diameter bullet.
Or, go all the way and rechamber for 8X57R and still have some factory loads.
I don't think that cutting barrels will regulate. Only screw up the ballance of the gun. Load development and experimentation will bring it closer. Round balls in the Shot barrel. Different speeds and weights of the rifle bullet. Start with the rifle and see what shoots to the express sights. Then work the shot barrel.