I'd have to dig it out and measure, but I think it's 20". I've never had a cylinder shoot the gap to another, but I'm not going to bet my fingers on it. I just remember not to put my left hand past the trigger guard.
In my zone, you can use a muzzle loader, shotgun, or handgun with a straight walled cartridge. Technically, the revolver is a muzzle loader because each cylinder opening is considered a muzzle. My deer hunting days are pretty much over, but I still love shooting it. Nothing quite like the smell of black powder burning.