A .32 muzzleloader will probably shoot either a .310 or a .315 patched round ball.
.310 roundball = 44.8 grains
.315 roundball = 47 grains
If it will shoot accurately with a 10 to 15 grain powder charge, you essentially have a muzzleloading .22.
A round ball slows down much more rapidly than a .22 bullet does so by the time the ball goes 50 yards, it's probably down to .22 velocity even with the hotter charges.
IMHO, the "squirrel rifle" calibers are some of the most fun muzzleloaders to shoot. I believe 00 buck works as roundballs for a .32 and is much cheaper than the roundballs marketed for muzzleloaders.
I shoot Hornady 000 buck in my .36 and it works quite well and costs about $26 for a five pound bag.