I made up a second set of barrels for my savage 24. It came to me as a .22 WMR over 20 gauge and I made a second set of barrels in .22 LR over 30/30. I had to bush the firing pin but that was the only action work. Now getting the two rifles to shoot to the same POI with one scope, that has been an ongoing project for years.
My goal was to have the .22 zero at 75 yards with the 30/30 going 3" high at 100 yards. One difficulty is self-inflicted, I keep changing my mind as to what load I want to shoot out of the 30/30 and if you change loads it means starting all over with the barrel regulation. It's a lot easier if one barrel is a shotgun since you don't have to hit a wabbit in the eye with a shotgun.
You could just leave the double as a shotgun, install rifle sights and shoot slugs. You'll likely still have a problem with regulation but by trying out a variety of slugs in each barrel you may find a combination which shoots close enough to be effective, and you'd still have the option of birdshot and buckshot when you like. Even if the slugs prove accurate out of only one barrel it still makes an effective combination without the weight of most true rifle/shotgun combos. When hunting wabbits during deer season I load my 20 gauge O&U with a slug in the bottom barrel and an ounce of sixes in the top, that's a very versatile combo.
