The Civil War era Minie ball was a hollow base bullet that was designed to be loaded from the muzzle, and to bump up upon firing. Lee Precision sells a mold in .578 that might bump up to fit your barrel, and it would be just right to load with the unjacketed part way out over the rifling of a .580 bore. You might want to look at the picture on Leeprecision.com and see if you think it would work. $26 for a mold, handles included, beats the price of custom work.
I don't know if this will work, as I have never tried it, but I don't see why it would not.
