I know Del Whitman of Lake Ann MI has done it, but it is neither as cheap or as easy as you might think to change to rifle barrels and regulate them to shoot to point of aim. Have you considered working up a custom slug load? A double rifle is inherently a short range gun in any event and you ought to be able to get usable slug accuracy out to 75 or 100 yards with plenty of knockdown power. Beyond that, use a single barrel rifle, lever, turn bolt, or SS. If you really have a need for a buffalo or elephant stopper, pay the money for a big bore British or European stopper because you are betting lives on the gun.