Without knowing the volume of barrels this project will require, you might want to talk to Ed Rayl who custom makes barrels currently for muzzleloading firearms. These would just be the tubes properly dimensioned on the interior. He currently works in chrome moly for his rifle barrels. Last rifle barrel I got cost a little less than three hundred dollars which included a lot of grinding to give it the octagonal swamped exterior which the shotgun would not require.