Anytime I purchase a traditional rifle, I would rather have a .54 caliber over a .50. They have better down range energy, and some claim better long range accuracy. I could never prove that. The .54 caliber is a great caliber and a roundball with 100 grains of powder will take out about anything you want to shoot with it. Put that ball in the rifle place from a reasonable range, and you will take that animal.
I also have shot a lot of deer with a .54 caliber. Only once, did I not get pass through. Why that one time was, after the ball entered the deer it hit a rib on the off side and started to follow that rib up and then around. Talk about taking a deer out fast.. but it did not exit like normal.
Also a Lyman flintlock is a great rifle. Even if you do have to shoot a left handed one..