The Cabela's guns are designed to be used to hunt Upland Game, using shot loads, not RB or Conicals. The newest versions come with some kind of Choke, making it even more difficult to choose a RB or Conical that will fit both barrels. This is NOT a Double Rifle, for African Big Game. These are smoothbores. The 12 ga. would be considered a small gauge for hunting big game by African standards, even if its a modern supository loader using 3 1/2" Slugs.
I own a 12 ga.DB shotgun by a different maker, but shoot it with 2 3/4 drams of 2Fg powder behind 1 1/4 oz. of #5 shot for pheasants, and rabbits. It patterns very well, for a cylinder bore gun. If the gun were choked, the gun would pattern even better with a modified choke. I keep shots with my gun under 35 yds., and have no trouble killing birds with it. If hunting quail, or close breaking grouse, I would switch to either #7 1/2 shot, or #8 shot, with the same powder charge and charge weight of shot. I could reduce the amount of shot to 1 oz. and expect to get good enough patterns to kill the birds if I got on them fast enough.
IF YOU INSIST on using RB, or conicals, you need to measure the bore diameter, to a thousandth of an inch, for each barrel, and then order components to fit each barrel. If you are Lucky, the Bore diameters will be close enough that you can use just ONE BALL diameter, and therefore only order ONE BALL MOLD> If the barrels are choked, you will need to know how much constriction is in each, and the ball and patch should be able to slide through the choke . Think in terms of a .685"-.690" diameter RB, and a .020" patch, for a .729 bore diameter.
If the muzzles are choked, then you need to consider using just Over Shot Cards, in lieu of thicker OP wads, or cushion wads, which will rarely fit down through the chokes, without being damaged. The OS cards are thin enough to be bent a little to fit them through the choke, and then turn them with your loading rod so they go down the barrel at right angles to the bore. The whole purpose of using wads is to seal gases behind the wads. If you ruin any edge, loading, it won't do that job well. :thumbsup: