The 1860 army cap-n-ball comes in 36 and 44. The 36 is .375 ball and the 44 might be any one of several. If you can get an ACCURATE measurement of the cylinder mouth, that plus a thou or three is your ball. Because the revolver doesn't use patching on the ball, the ball must press fit into the cylinder.
There are three reliable ways to prevent chain fire:
1. Lube over the ball with a thick grease, which won't start to run under its own weight until it reaches above a temperature of ~150F. That said, know the temps you will be shooting in and add 30F. Naturalube and Wonderlube work fine most of the time. August in Death Valley are not friendly to them.
2. Use a lubricated overpowder wad of the right dimension. This works very well, but only if you are going to shoot right away (24 hours.) I prefer this, although it raises the cost of each shot slightly. I've had trouble with the lube from previous shots capturing a few grains of the new load and building up a chain around the cylinder mouth. This is the real cause of most, if not all, chain fires.
The reason bio lubes are recommended over petrol lubes is because they will penetrate the pores of your cylinder and barrel and form a super-hard coating of carbon. This is called seasoning. If you do any cast iron cooking, you know what I'm talking about. This seasoning is just about impervious to sulpheric acid and protects your bore much better than all the base sprays on the market. You still have to clean a black powder weapon to the old "once for" rule, but you will be better protected when you have hours between you and boiling water.
ONCE FOR rule:
After firing a black powder weapon, clean it with boiling water with just a drop of soap until patches come out clean. Then, using a good non-petrol cleaner/lubricant:
Clean it once a day for a week
Clean it once a week for a month
Clean it once a month for a year
Clean it once a year forever.
With any luck at all, you'll never reach the once a month, because you took it out and got dirty, again.
Test on Friday.