Many years ago I made a 5/8 dia ball mold....the balls were for a slingshot. Made 2 ground blocks, laid out the centers and drilled a blind hole short of the ball bottom. Then used a ball cutter and intentionally went deeper than the half ball depth. Used a 5/8 dia ball bearing to measure the half dia depth. When the cavity depths for both mold halves were correct, the ball bearing was clamped in the 2 blocks, the outside surfaces of the blocks were aligned and the guide pin holes were drilled.....the holes for the pins were a press fit and the other blocks' holes were a slip fit. The necessary shapes and holes for attaching the blocks to the handles were finished and a minimum protruding sprue cutter was made and attached. The lead balls needed no further attention.
The reason for the 5/8 dia balls was that they were visible in flight whereas a 1/2" dia ball was faster but wasn't visible in flight. This was important when shooting at longer distances. .....many times the rabbit wouldn't move after the first miss and the 2nd shot would kill because of a possible adjustment. Actually w/ a lot of practice, these slingshots were deadly on small game.