An upsidedown wad, as Yoogi put it worked for me - well indeed. I had an undersized mould, .684" and was shooting them in a .725" smoothbore. I cut the petals and supports off spend trap wads, then loaded cup-one down on the smokeless powder as a gas check, felt or fibre wad, then a base cup, cup-up underneath the ball. The plastic wad kept the ball in the centre of the bore and gave excellent accuracy. In a rifled barrel, the cup would transfer the spin to the undersized ball and theer would be no leading as the ball would not contact the bore itself.

With this in mind, an undersized ball could be used in the .20 bore, ie: .575" through .615". Moulds are available from various mfgrs. in .575", .580", 590", .595", .600", .610" and .615". I think there may also be a .620" and perhaps a .625". These mfgrs. are Lyman, Lee, RCBS(maybe), Saeco(maybe) Rapine and of course, Jeff Tanner who will make any size you want. Price wise, Lee are the cheapest and Jeff's excellent brass moulds are next in line. RCBS are the most expensive at around $124.00 with Rapine next lower by about $40.00. Different places have different prices. The Lee moulds run around $14.00 to $19.00, while Jeff's moulds are in the $30.00 neighbourhood. Lyman's run around $60.00 in the States, some $25.00 below Rapine's offerings.

I think that twist is way too fast for a RB, butt he proof is in the shooting. Some guns will allow it, some won't. I'd prefer a minimum of 60", and 80" would suit me even better. You can get muzzleloading barrels with these twists, but I don't know about 4140 steel barrels with these slow #'s.