Measure the bore of the barrel with calipers to know for sure what you have. Then you can order the right diameter wads. I use 12 gauge wads from circle fly, but I obtain them from whatever supplier is handy.
MY 12ga. is a cylinder bore, and I use a load I got from an article in The American Rifleman magazine, repulishing an article printed by its predeceassor publication 100 years before. It was a " Favorite 50 yard load " of Commercial Duck Hunters on the Illinois river, back when that was legal.
2 3/4 drams of FFg powder( 76 Grains), and 1 1/4 oz. of #5 shot. I was using a standard OP wad, followed by a lubed( moosemilk) cushion wad made of celotex, 1/2" thick, an OS card, the load of shot and another OS card to hold the shot in the barrel. I get dense enough patterns to cleanly kill pheasants at a measured 33 yards.
I occasionally get a cushion wad following the shot, and opening a hole in the pattern. I am trying Jim Rackham's recommendation to use ONLY OS CARDs as wads. 4 OS card over the powder, the load of shot, and 2 OS card on top of the shot to hold them in. His pictures clearly indicate that this combination eliminates the " donut hole " patterns, and gives nice, even shot distribution within the pattern.
For Denser patterns, I am trying paper shot cups, and Britsmoothy has advised how to make them, and got much better patterns from his cylinder bore gun. I believe its possible to get Modified patterns out of my Cylinder Bore gun, using the cups, and that can add another 10 yards to the effective range of the shot, provided you use heavy enough shot.
I hope that helps.