490 ball and a 15 patch is a good place to start. Patch thickness is one of the variables you should experiment with to find the load your rifle is going to be most accurate with.
I have three 50 cal rifles I am running now. The CVA mountain rifle shoots best with 490 balls, 15 patches and 70 grains of Goex 3-f. The Lyman GPR likes two loads; 495 ball, 20 thick denim patch and 40 grains of Goex 3-f is the best target load and 495 ball, 18 thick pillow ticking with 80 grains Goex 3-f is the best hunting load.
The little CVA Frontier rifle is a tack driver with 490 ball, 15 patch and 30 grains of Goex. This is my youngest boys target and small game rifle, we haven't worked up a load for deer with it yet.
The groves in my Lyman rifle are cut deeper than those in my CVA rifles. It needs a thicker patch to fill those groves and make a good seal.
You will just need to try a few combinations until you find the patch /ball combination that gives you a good gas seal. If you are seeing fired patches with cuts and burned through signs, try a thicker patch.
Don't forget there are 480 round balls available. I have friends who shoot those with 20-22 thick patches and they can be very accurate in some rifles.
Muzzle loading rifles are spooky accurate with patched round balls if you take the time to experiment and work up your load.
