A black powder pistol ... if it is a single shot like a Trapper Model, Kentucky Model, etc.. get as big a caliber as you can if you intend to hunt with it.
A black powder revolver holds six shots. I have a .44 caliber Pieta Target model and it is very accurate, but whether I would want to hunt with it, I am not sure. I would have to be real close to do some shooting with it. I have hunted rabbits with it. Now that is a challenge.
Revolvers come in a variety of calibers, .44, .36, .32, and some custom guns even less then that. Just be aware a good load for a revolver is 25-45 grains depending on brand.
If I wanted to hunt with a Revolver I would go to a Ruger Stainless black powder revolver. They accept a higher powder charge and are top of the line weapons.
If all I wanted to do was plink, my Pieta is just fine. Since mine has target sights, it is very accurate out to 30 yards.