That is a big pluss for the SxS double. You can load a tradegun for anything you may encounter but you have to know what you'll encounter before you load. A double gives you two guesses!
I've found proper wads under the ball to be far more important than the patch. Ball loads are the only time I bother with a "cushion wad". A thin card wad under the ball is useless as the gas pressure just wraps the thin card around the back of the ball, making the card too small to seal the bore. Load a .125" nitro card or two, followed by a thick fiber cushion wad, then the ball patched in anything thick enough to hold it snug. The cushion helps to keep the roundball round and to keep the over powder cards flat so that they seal the bore. Buy REAL wads from Circle Fly, not those wimpy little "wonder" things. The patch only needs to hold the ball down snug atop the wads and keep it centered in the bore with no "shake, rattle and roll" as it rapidly departs. Both the patch and the cushion wad can carry lube.
Since the smoothbore ball has no rotation it is of prime importance that the ball be as near perfectly round as can be. Cast them of hard alloy so that the force of acceleration does not flatten the back side and file the sprue off the front as neatly as you can.
The ball weighs about 1 1/8 oz. so you can just use the same powder charge as your shot load, at least for starters.
12Ga. balls pack a whallop, on both ends of the gun! :m2c: