.50 works, .54 hell of a lot better, and .62 is better yet.
.50 & .54 round balls like to flatten out if you hit in the shoulder blade, .62 goes right on through. Has been my experience anyway. I shoot for the heart lung area with round balls. Hornady Great Plains bullets in .50 or .54 will break a shoulder on Elk.
Flintlocks and Patched Round Ball, hell of a lot of fun.
Slenk