Reaming an old rifle barrel to smooth bore is a great way to repurpose a neglected barrel. Keep in mind that it won't be a sky buster for geese. Generally the largest you can go is 54 to 58 caliber. trying to swing all that weight for pass shooting is quite a task. Consider that shot guns have heavy breeches and thin barrel walls at the muzzle. It creates a natural sighting plain that accounts for drop of the shot pattern as well as substantially lightens the front of the barrel for swinging after a dove or quail. It will work just reamed, but it will be easier to aim and carry if you have the forward end of the barrel tapered to shave off weight. I have done this by hand with files, taking the forward two thirds of a 34 inch barrel to round, and I have done it on a lathe. Frankly, the hand method keeps the octagon to 16-a-gon to 32-a-gon to round a much nicer appearance. I filed the corners starting about 1/4 way up the barrel making gradual additional flats so by 1/3 of the barrel it was 16 flats and then from there to half way up the barrel i gradually filed the 16 corners to look like 32 flats at the half way point. the forward half of the barrel was filed round and slightly tapered to the muzzle. That was only a 7/8 ACTF 50 caliber smooth bore, but it made the barrel much better balanced and looked pretty good on a half stock. I epoxied a thin barrel rib under it. Lost that gun when my shop burned.