This is a project I made years ago. it is a swaging die for a .45 cal round ball. A pre cut slug was placed in the cavity then the die was closed as far as possible. The die was struck with a heavy soft faced mallet. A relief was around the ball cavity to allow some place for the flash to go. The ball was removed and pressed through a flash removal die. the process was somewhat labor intensive. It produced a beautiful ball.
Testing was done using cast balls vs. swaged balls. There was no noticeable difference in accuracy from my left handed flint lock Tennessee rifle. Both types of balls shot to same point of impact. After shooting all balls produced I felt that cast was the easier way to go.
The die was made from O-1 tool steel and hardened to about 56 Rc if my memory serves me right. The die also serves as a cutter to cut slugs to proper length by pivoting on one of the pins. It was a worth while experiment even though it is seldom used any more.