Thread: sleveing a 12ga to 20 or 28ga

hi there i brought a 12ga over under it has 2 sets of barrels but one has a small bulge just infront of the chamber the guy i brought it off sait that he was going to sleve the damaged set if barrels to 20ga
i thorght that would be quite cool to be able to change cals just by changeing barrel sets

any one got any idea of cost involved who would do it where to get sleves etc

cheers