You are talking about a Minie ball which has a hollow base to its shape, no grease grooves, and a nose heavy projectile because of the hollow base.
Think Badminton birdie, made of lead.
I don't know of any made for a .50 cal. rifle, much less one that would work in a deep grooved barrel set up for shooting PRB.
You might be able to find a custom mold maker to make you some thing like this.
T/C makes it Maxie ball, which has a flat base- not hollow- and grease grooves, with bands that help to center the short bullet in the muzzle. But, this is NOT a Minie ball, which is what you have asked about.
Sounds like you have cast enough balls to let you practice your shooting skills for a few months, so that you become a better shot. Dry fire at home, and practice fundamentals at the range. The practice will serve you well no matter what gun you fire in the future. :thumbsup: