If your bores start out at say 1" apart on the centres, and cross at 75 yards, then at 100 yards the seperation of the projectiles is going to be 1/3". (1" at the muzzle, .66" at 25 yards, .33" at 50 yards, 0" at 75, 1.33 at 100 and so on till at 150 they will be 1" apart, but on the opposite...