Im going to give this question a whirl, basically when you shoot at a downward angle, you are using the force of gravity to your advantage creating less drop in the bullet since it is already going downward. As opposed to some1 shooting perfectly horizontal with the ground, the bullet would gradually drop until it hit the earth making more of an arc than a straight line. A bullet that is already being shot downward doesnt have to arc becuase it is already heading toward the force of gravity, therefore making the bullets path more of a straight line than an arc. So it would seem that if you shoot at a downward angle, you would have better accuracy becuase there is less drop in the bullet over distance.