Something is not right.

If I understand the gun's bore, it measures .594 across the lands, and .625 across the grooves. This means the rifling is .0155 deep.

If that is correct, a correctly sized "mini" type bullet would be right around .594 +/- a couple thousandths. That style bullet is made to slide right down the bore without engraving. The hollow base expands into the rifling when the gun is fired.

I'm not an expert on roundballs either, but I do believe the ball itself needs to be slighlty smaller in diameter than the bore diameter (measured across the lands).