Both bullets I've cast measure out at .577 and either, unlubed, will drop into the barrel -
If they just "drop into the barrel", that's a huge red flag. A minie that is close to the correct size will not do that. For a "drop in", your windage is at least .005+ and maybe more and that is in no way conducive to accuracy. From your description, I'd bet the bore is .580+.
To get a repro, or original, to shoot well, you must be within .001 of the actual, measured bore size. I have a friend, and N-SSA team mate, that shoots an original LG&Y and it takes .580 to shoot well and it's as original as they come. With a .580, it will shoot most style minies very well and some more than others.
You can get pin gauges on Amazon for about $5-7 ea and you only need a couple. You know that your bore is over .577, way over, so I'd suggest starting with these pin gauges- .578, .580 and .582. If .578 drops in and .580 won't, your bore is .579ish and that's close enough. If .580 drops in and .582 doesn't, your bore is about .581. Do NOT attempt to force these when measuring. It's just a finger held, slip fit.
Once you have that dimension, order a sampling of minies from here and HERE ONLY-
https://www.lodgewood.com/Bullets_c_7.html
Order a selection based on your size and be sure to specify the size needed. To answer a question before it's asked- Lodgewood is the ONLY source I trust for commercially cast minies. Their minies are cast by a N-SSA competitor who knows what he's doing. Stuff from a round ball place like Track or Log Cabin is hit or miss, mostly miss on quality. They may know round ball, but the guy casting for Lodgewood is a specialist who is very good on quality.
Depending on where you are, I'd also recommend getting to know some N-SSA folks. We compete with these critters in live fire competition and know what it takes to get one to shoot.
Repro 1862 Colt Contract. Flyer is from a broken nipple. And yes, the nipple is a wear item. Keep spares on hand.
This group is 10 shots, rapid fire, offhand at 50yd from a "Zoli" Zouave. Even most repros can be made to shoot well with some load development.