Hunt...With a round ball, forget the energy levels that modern gun writers talk about...A round ball kills by punching a half inch diameter hole (or so) through both lungs...If the ball expands, that's fine, but it doesn't have to in able to kill effectively, in fact, too much expansion leads to less penetration, so with a pure lead ball you can have too much velocity....

When I bought my first flintlock in the mid 70s, I didn't know anyone else that shot the dang things...It was a .45...I found that 75grs of powder would penetrate through a 2x4 at 75 yards, I figured that was good enough for deer...
For squirrels, I dropped down to 45 grains and turkeys 50 grains...The first deer I shot with that gun, the ball broke both shoulders, went through the spine and was under the hide on the off side...

I killed about 25 deer with the .45 and only went to a .54 when I had an encounter with a bear on one of our farms...Even with the .54. I use 80grs of FFF, it's plenty for deer out to 125 yards, which is as far as a man needs to shoot with open sights anyway...