Clayman: I don't know where you get the idea that we don't use lead shot in the USA, but you are very mistaken. We just can't use it to shoot Waterfowl, like Geese and Ducks. Other migratory birds are shot with lead shot, and nesting birds are all shot using lead shot. The size of the bird pretty well dictates the size of the shot used.
I don't understand why you don't use tight wads over your powder charge. You are losing velocity to gas leakage when you don't use a tight wad. I started with my fowler working on a round ball load, as it make it easier to check the loading components, using a single projectile, than using shot. I quickly learned that my barrel was oversized, and I ordered 19 gauge wads to use in my " 20 gauge " gun. I had an immediate increase of 20% in velocity over what I was getting when I used standard 20 ga. wads in my gun.
Iron Jim Racham has now written several times here on the forums about using only overshot (OS) cards in his gun. He uses 4 OS cards, instead of a 1/8" thick overpowder card wad and a cushion wad, under his shot loads. He uses two OS cards on top of the shot. Here's why. He puts holes in each of the thin cards out towards the rim, and then orients the cards at 12, 3, 6, and 9 o'clock as he loads them down the barrel. No hole in a card allows gase to escape because they cover each other. The two card on top of the shot are used so no small shot, if he uses this on clay targets or dove, for instance, will pour through the hole.
The holes in the cards all air to separate the cards as they leave the muzzle, so that they fall to the ground quickly, and do not follow the shot downrange. Those of us who have used the traditional hard overpowder card, then a cushion wad soak in moose milk, or lubed, then shot, then an OS card, found that the cushion wad weighed too much and would blow a hole in the pattern by following it out the barrel. We have experiemented with use only 1/2 or 1/3 of a cushion wad to reduce that weight, and stop the wads from following the shot and blowing the patterns, with some success. Jim Rackham seems to have a better idea. At least you don't have to carry three different wads to load your shotgun. I was already using an awl to put a hole in the center of my OS cards so I could get them down on top of the shot in some reasonable amount of time. Putting holes to the outside of the card so I can arange the cards like Jim recommends is a minor change in my pre-shoot plans. Leaving the other wads home is going to be a blessing.
YOu might want to read Bob Spenser's Black Powder Notebook to get great loading information. In my 12 gauge shotgun a 2 dram load just doesn't produce very good patterns. I am going to retest all the powder charges with Jim Rackham's technique, to see if its the powder or the wads that gave me the indifferent patterns. In my 20 gauge fowler, I see no need to use more than 2 3/4 dram of powder and 1 1/4 oz. of shot. Most of my upland game loads are in the 2 1/2 dram/ 7/8 oz range.