John, there's one problem which no one has addressed here. You can't screw in two rifled choke tubes, screw on a set of rifle sights, and expect that your barrels will print together, at any range. The only way that you'd achieve that would be to re-regulate the gun. What you can do is to put together a 12 gauge cape gun. Rifled tube in the left barrel, sighted to the sights. A heavy load of "00" buck in the right barrel. No shots beyond 20 yards with the buckshot and it's devestating. The slug will do out to about 75 yards. You can't do this with a 20 bore because the 20 just won't carry the mail with buckshot.
