To obtain the best results deflecting armor should have a hard, smooth surface, and possess sufficient rigidity to turn the ends of shot and bring them sidewise against it; the great penetration obtained by elongated shot is due to the concentration of energy on the small area of their cross section. The area of a longitudinal section of a shot of three calibres' length is almost four times as great as the area of the cross section, consequently the penetration of such shot would be proportionally reduced if they were made to strike on their sides. If the armor does not possess a hard surface, the local give will permit the ends of the shot to bite and the armor will be penetrated. Armor of a proper thickness, having a hard, smooth surface, disposed at an acute angle, will possess considerable elasticity when struck by shot, by yielding bodily to it. Even if the entire energy of shot is absorbed by armor, if the shot is made to strike upon its side, it could be resisted with a moderate thickness of armor.
Professional Notes: Deflecting Armor
N. B. Clark