NB: I'm assuming you've found your case mouths too small to accept a readily available .575-.580" minie bullet? The easiest route is to use those after flaring the neck and squeezing in the bullet with dies. A lot of guys don't seem to want to buy the dies but I would recommend it if you plan to do a lot of shooting and perhaps acquire more than one Snider. If not, you might consider finding something conical in your toolbox that you can use as a hillbilly flaring tool to open the moth up enough to take a .60" roundball—which I believe you can buy easily enough. I know a number of guys who have fireformed their brass that way. Back to your original question—.570" isn't a standard size for bullets or balls (as far as I know).
milsurpo