Finally got off my duff and ordered some supplies. I ordered some .020 patches to try with the .57 ball. Still a little shy of .615, but might be good enough to use up the the .57 I bought on impulse. Also ordered .010 patches and .595 ball, as this would seem to be the magic math to reach the diameter.
Will post when I get a chance to mess about, but it might be some time.
Any owners of this double barreled monstrosity have any max load advice? The manual states 28gr for shot, 30gr for ball. These seem awfully wimpy, but the barrels are not overly thick either...