as an old "redleg",i.e. former ARNGUS artillery officer for 2 years, your post makes no sense, given the primative sort of artillery available in 1860s.
free dixie,sw
Your post was actually reasonable this time, so I will respond. A diagram would make it easier to explain, but here goes:
The artillery should not have been directed perpendicular to the Union line but at less of an angle, say 70 degrees, with half the cannons pointing to the left of perpendicular and half to the right. This way, a much greater length of the Union line would have been in the path of each shot, giving over and under shots more chance to hit the Union line. It is precisely because of the primitive sort of artillery available in the 1860s that this tactic should have been adopted. Competent artillery officers did it all the time during the Napoleonic Wars and the Civil War.