A heavy load would probably be 1 1/4 ounce of shot or more. That would most likely include all buckshot loads. The light loads are the 1 ounce or 1 1/8 ounce of shot that are intended for clay target shooting or small birds like dove, quail, and grouse. Any shotgun shells specifically for ducks, geese, and pheasant are probably going to be at least 1 1/4 ounce of shot.