I can give you a possible reason for the large difference in the points of impact. A slower, not faster, bullet will usually(of course up to a point) shoot higher. I know this seems nuts but if you think about it, the slower moving bullet is in the barrel longer an is subjected to more rise of the gun from recoil. I know this to be a fact using a hand gun, the slower load always prints higher. I also found this info in a reloading manual.