Track of the Wolf has a wide selection of rifle liners. Others do as well. You can get a lot of information on thefiringline.com forums and other rifle oriented boards. Low pressure cartridges are a must for this type of alteration. The liner is the real barrel, that is held in by solder or epoxy. Any voids in the epoxy creates a weak spot. Solder never goes from end to end so it has even more voids than epoxy. Too high pressure would cause the liner to distort which would ruin the liner for accuracy. So you can not place a .225 liner in your double gun, the pressures would be too high. Select your caliber according to this type of limitation.
Before you go down that road I would try the barrels as they are. You are looking for a plinking gun not a match rifle. Hand loading could help. I have been thinking about taking one of the Rook rifles that has been converted into a shotgun and have them relined back to a rook rifle. Economics of the job just makes it a money pit but that has not always stopped me before.
