The height of the rib above the bore does not correlate with point of impact. It's the angle between the rib and bore, coupled with comb height. High rib trap guns allow for a more erect head position, and are thus easier on the cheek. It's the comb height that results in a high shooting gun.
Jim, you would be correct if lead in the vertical plane was all we needed. Then, a fast swing through would solve the problem of a low shooting gun. Consider a crossing target. With a low shooter, one must swing through the bird, losing sight of it while passing... either that or use a sustained lead or perhaps start the swing from a point in front of the bird, all viable techniques. Shooters do all this, some better than others.
Being an old trap shooter (and getting older), my shooting is mostly swing-through. A high shooting gun within reason is preferred.
