Hunting with a Muzzle loading shot gun can be fun and challenging. I use a Thompson Center New Englander 12 gage single barrel. You only get one shot at a time with this gun. You can always use a double barreled gun to to get the second shot before reloading. But I like a lite gun and it gives the game just a little better chance.
If you are looking for a gun and have not bought one yet, I would lean towards getting a 12 gage gun. You can find 20 gages but you will only be able to load a 20 gage load in them. You can always load up your 12 gage with a lighter load and go up to a full 12 gage in the same gun. This way you will not have to get more than one gun when you want to hunt bigger game with your 12 gage. Besides a good double barrel will set you back $ 750.00 or more new. They also make 10 gages also. Used for larger game, like turkeys. But the 12 gage will hunt them just the same. You might have to get a bit closer to you game.
The size load and size of shot (pellets) you use will do a lot with what you plan to hunt. Rabbits you can use a lite load and #6 shot than say squirrels. I have knocked squirrels right out of a tree just to have them hit the ground running. A larger shot size #4 shot would be good for them. If you hunt birds I would use #8 for dove and #6 to #7 on grouse and #5 to #6 on pheasant. The smaller the shot size the more shot you have in your load. Makes for a denser shot pattern. But it will not carry as fare as bigger sized shot. I will mix shot size in the same load and have had good luck that way too. But I would always take the gun out and shoot news paper to see how the pattern is at different ranges. I once got a squirrel at a fare distance that only one of the larger pellets hit it in the chest cavity killing it instantly.
I always fancied Top treestands for hunters where you could sit back and wait for a potential target but I was appalled at killing the poor squirrel because I was quite fond of them and the fact that they are small defenseless creatures made it all the more agonizing.
Powder loads for a 12 gage can go as lite as 70 grains by volume to as heavy as 100 grains. These are the loads recommended by Thompson Center for the New Englander 12 gage. Other manufacturers may recommend something a little different. Shot load is measured in the same volume as the powder to keep things simple. 70 grains in a powder measure will give you around 1 once of shot by weight. 80 grains will give around 1 1/8 once, 90 grains 1 ¼ once and 100 grains will yield 1 3/8 once. Also the heavy hunting load will give the best shot patterns for the bigger sized shots.
Always use good muzzle loading practices when hunting or just target shooting. Never smoke around black powder. Always wipe out your barrel after every shot. It is not worth you losing your eye site, any other part of your body or life just to have it go off after you dump and new load of black powder down the barrel because you still had a live spark in the barrel. When using a double barrel, use a full sized wad on top of the shot. This will help the unfired barrels load to stay tight after shooting the first barrel. When re loading the first barrel re tamp the second unfired barrel again too.