That is too much powder. Stop thinking like you are shooting a modern gun with smokelss powder loads and choked tubes.
Read the V.M Starr article on Shotgun loads on the
Bob Spenser's Black Powder Notebook.
[url]
http://members.aye.net/~bspen/index.html[/url]
Compare to his article above on the site. You will get better results if you use a shotcup of some kind. In 12 gauge, you can use the modern shot cups, and they will improve the density of your patterns, and extend your range to 30 yards and possibly out to 35, if you use the correct shot size. I recommend #5 plated shot, 75 grains or less of black powder, and no more than 1 1/4 oz. of shot. YOu can add more shot- up to 1 1/2 oz to see if that adds more pellets to the core pattern, but I don't see enough improvement to warrant using all that extra shot.
You can use paper cups, but make them out of heavy paper-board( like index card stock) only thicker if you can find something. You will need to form them around a mandrill, and you can make that from wood dowel, or even pipe. You can glue the seams, and tape the bottom. Try finding 20 or 16 gauge card wads to put in the bottom of the cup to give a firm surface. Or you can use a couple of OS cards , but you will need them also in a reduced size, so that the entire package will slide down your 12 ga. barrels easily. Cut slits in the sides of the cups, just like the plastic ones have, so that when the "Petals " hit the air outside the muzzle, they open up and allow the shot to separate from the cup. Never make the cup longer than the amount of shot, as the ends will fold over your shot, and created a shotgun slug that is frangible when it hits. Us a standard OS card to hold the shot in the barrel.
The cup protects the outside pellets from rubbing a flat on them as they travel down the barrel. That helps them keep their shape, and stay in the pattern. Try to keep the shotcup a lose but close fit to the bore of the gun. Use an Over powder card, or a couple of overshot cards on top of the powder to seal the gases. Roundball likes the felt Wonderwads, but I am not impressed with the consistency of velocity they give, over more firm materials. I believe that gas blows by the felt wads, when it doesn't with the Overpowder, or OS cards. At least that is what the chronograph is telling me. Finally, consider switching to FFg powder, as it is more foregiving, and it wil recoil less. That does not matter much on a hunt, but it does when you are on a range, trying to learn how to shoot that shotgun.