Be nice to the gun. Start using 60 grains( 2 1/4 Drams) of FFg powder, then an OP wad, then a 1/2 Cushion wad, lubed on the outside edge only, then a 1 oz. load of shot, followed by an OS card. This is a light load for a 12 gauge, but Until I had fired that gun many times, I would not increase either the shot load, or the amount of powder used. Read the two articles on Shotgun loads written by Bob Spenser, and V.M. Starr, on Bob Spenser's website,
http://home.insightbb.com/~bspen/
You will learn alot. Then go back and read the last couple of years of postings on this thread and forum.
A fEw years ago, Iron Jim Rackham posted several comments on a load he was using in a New England Style Fowler he had Mike Brooks make for him in 20 gauge. He used ONLY OS cards, putting 4 cards on top of the powder charge, then the shot, then 2 os cards. All the cards had off-center holes poked into them to allow air to flow through the cards to prevent Dieseling when he was trying to load his gun. Using only OS cards allowed him to go hunting, without worrying whether he enough of or the right combinations of wads for his gun for each shot. He posted pictures of his shot patterns, which were impressive. He also took several Grouse, which he proudly showed off. Its a different way to load your gun, but it also seemed to work quite well for him.
Oh, Jim put a small "ball" of lube between his 3rd, and 4th OS cards on top of the powder, so that the lube would be forced out to the sides of the card to lube the barrel as the shot was fired. I don't like having to carry little balls of lube, made of wax and oil. So, I prefer to grease my barrel with a lubed cleaning patch in front of the jag, when I run the two OS cards down the barrel and seat them on the shot. The cleaning patch comes out with the rod, and jag, and the lube protects the bore from rust during a hunt, while letting the lead pellets glide over the greased surface of the barrel, rather than leaving lead streaks on the walls of the bore, and flats on the lead pellets. The remaining lube softens the BP residue that follows the load, making it easy to clean before the next load is poured in the barrel.
Wads: OP wad= Over Powder Wad. I use Walter's Vegetable Fiber Wads, which you can by from Track of the Wolf, and other suppliers, including from Walters.
Cushion wads, can be bought from Track, and come in various thicknesses. Mostly they are cut from 1/2" thick celotex, a building material. I split the wads in 1/2 or even 1/3 to reduce the weight. Roll the edge of the wad in vegetable oil in a saucer, so you don't get too much oil in the wad.
Reducing the weight keeps the cushion wad from following the shot down range, and actually bumping the back of the shot column, cause a hole in the center of the pattern in some guns. The faster the velocity of a load, the more likely you will get a "donut hole" pattern if a thick wad is used.
OS Card= Over Shot Card, again available from Track and others. These are thin slick surfaced cards cut from Shirt Backs. The harder, slick surface is good for pushing the shot out the barrel, and for catching the air out the muzzle so that the card quickly drops away from the advancing load of shot.
My standard hunting load for my DB 12 gauge shotgun is 75 grains of FFg, and 1 1/4 oz. of #5 shot. My barrels are cylinder bore, meaning there is no choke in them. There are, however, several ways to gain some choke effect in such a barrel by how you load the shot in them. BP shotguns, with Cylinder bores, are generally considered to be 25 yard guns, and folks are discouraged from attempting to take birds at much further distances. Some barrels have enough metal in the front end to allow " Jug Choking".
Just remember that if you do not use some kind of protection to prevent the lead pellets from leaving lead streaks in the bore, your patterns will suffer as the lead streaks build up. In addition to cleaning the gun with soap and water, you will also need to use a good modern Lead Solvent to clean the lead out of the barrel.
The most recent idea for a good "fix" of this problem is described below in a thread titled, .62 Colerain Turkey Choke barrels, on the index to this topic.