If shot size, velocity, and marksmanship are truly equal, then the answer must be in the choke effect of the different guns. 5/8 oz of shot simply can't be more effective than 7/8 oz or 1 oz if shot size, velocity, marksmanship, and choke effect are equivalent. Bore size of the gun has very little to do with this question --- other than trending in the direction opposite favoring the 28 bore gun.