Find a good load and shoot them in the head, any caliber works for head shots...
I would bet a .45 ball would work in a black .451 sabot and a .50 inline...

From 1977-1990 the only muzzleloader I had was a .45 caliber flintlock...With it I killed rabbits, squirrels, deer, turkeys, groundhogs, etc...My deer load was 75grs FFF, squirrel load .40 grs and turkey, groundhog was 50grs...

I finished making a .54 in 1990 and used it for squirrels a few times, worked fine with .50gr FFF...I later rebarreled the .45 to a .40...I use 25grs FFF in it for squirrels...