Be wary of the background when using shot loads in cap and ball revolvers.
Some years ago I tried it against a large sheet of paper tacked to plywood.
The first shot from about 20 feet, the shot bounced back from the plywood and struck me. It didn't have enough power to pierce my clothing or skin, but gave a loud "TICK!" to my glasses.
Wear eye protection when experimenting, and don't shoot straight on against any hard object.
I used a .44 Remington. Don't recall the powder or shot charge, but at 20 feet the pattern was about 3 feet across. The .44 chamber holds little shot, so there wasn't much chance of hitting a snake at that distance.
If I feel the need to carry snake medicine, I carry my .38 snubnose with Speer shotshells. Up close, a cap and ball might work okay but you'd probably have to be within a few feet for maximum effect.
I don't make a habit of shooting snakes. Even rattlesnakes have a right to live, as long as they pose no danger to me or others. If I see a snake, even a poisonous one, I simply walk around it.
Snakes keep the rodent and insect population down. If there is no dire need, don't kill them.