Just like sighting in a rifle, you need to try your shotgun with different chokes and different brands buckshot...Anything else is just speculation...

My Browning B-80 does best with Winchester Supreme 3 inch Mag 00 buck out of the Improved Cylinder choke...It will keep 5-7 pellets into a 8 1/2 x 11 sheet (turned horizontal) at 40 yards...