I bought a .575 RB mold from Lee--the ball seriously bulges my unslit BPI wad that I hoped to use. So, I bought some Hornady .570 balls, and they still appear to be too big.
I haven't messed around much with it. From the reading I did on Cast Boolits, the claims were you should be able to push the ball/wad through the barrel. It should be tight, but you shouldn't need to pound it through.
I have .575 and .570 balls, and a bag of unslit BPI wads, if you'd like to mess with it. The balls come in at almost exactly 5/8oz, and I was able to find some 5/8oz reloading data (I want to say, using Unique). I thought they would be some good close range HD rounds. I mean, it's a .58 cal round ball and that has centuries of real world data. I'm just not sure how tight is too tight?
I tend to think if the round ball can roll down the barrel without friction, you should be able to swage the plastic wad to fit without creating undue pressure. The unslit wads have worked perfectly for my 20ga "socket slugs". I actually sent those for pressure testing, and they come in within SAAMI.
I you want to pursue this, please keep in touch. It's a project I was messing with, but it got shelved when i started working with the "socket slugs". Even those, I've not tested beyond about 20 yards. In the unslit wads, the wad stays on the slug acting like a kind of drag tail. It's a cool idea, but I also couldn't find a lot of good 20ga data for the round balls. Anyway, I have several hundred .570 and .575 balls and a bag of wads if you'd like to try it.