Shot a bunch of 575" RB's out of an approx. .575" bored, Italian Enfield Musketoon. The .575" RB's I cast from a borrowed mound were just about exactly .575" all the way around, with a tiny sprues from a 4 cavity mould. They weigh slightly different, cavity to cavity, but for interest's sake, run 288.7 to 290.6gr. The lead I used is about as close to absolutely pure as I've ever seen. There is no temperature that Will allow the lead to remain uncoloured as it it still turns blue, gold, purple, etc even at solidifying temps. It is from the walls of an X-ray Hospital room. At casting temp, the oxidation is rapid and the dipper must be cleaned of the oxidation crust often - about every 4 to 5 casts with that mould, about every 10 casts with a DC mould.
My first .58 was a Zouave, which liked .575" balls and .020" denim patches and the second, an underhammer H&A(or whatever) so I used the same balls in it - Numrich barrel I presume, but it had ratchet rifling - a mistake, I also presume. Both guns liked the .575" and heavy denim patches. The third was a GRRW .58 1-1/8" X 34" tube on a Hawken Taylor made me - also liked the .575" and now the Musketoon also likes them but they're a bit snug as the ball won't drop down the bore naked, just sits on top.
I can only use a .015" patch with the .575's as the rifling is only .003" deep at the muzzle, increasing to about .010" at the breech - or thereabouts. I put in a very short, 3/8" cone at the muzzle and rediused the edges of the crown so they load easily, even though larger than the bore's actual size. Due to being 5 groove, it's hard to measure, I suspect about .5735" to .574" actual measure. All this to say if you cannot use a .575" ball in a true .580" bore that has normal .010" or deeper rifling, your crown is not radiused enough or not smooth enough. Factory crowns are cut with a machine tool with straight sides, ie: no rounded surfaces, so leaves sharp edges that prevent easy loading and swaging of the ball and patch. These edges need to be smoothed and rounded, then you can shoot a descent ball and patch.