Greetings All:
I have a Pedersoli Kodiak 45-70 double that I recently bought used. I am trying to find a regulation load for it, but am not having any luck. The individual barrels group well at 100 yards, but the groups are about 8 inches apart at that distance. Changing bullet weights and powder only result in the groups changing vertically, but maintaining the same distance apart. I read that Pedersoli used 300 grain Winchester loads for regulation. I have tried 300, 325, and 350 grain bullets with varying velocities without getting the impact points closer together.
I have read that the impact points will diverge if the velocity is too low, and will cross if too high. Frankly, I am having a hard time getting my head wrapped around this theory. I can see velocity and bullet weights changing impact point vertically, but how do they go side to side?

Thanks, Texas Bill