[1412]. Shot group and center of impact. If the cone of fire be intercepted by a target (for example, A O, [Fig. 44]) at right angles to the axis of the cone, the shot holes will make a pattern or group called the shot group, the holes being the thickest approximately in the center of the group, called the center of impact. From this point in all directions the density of the grouping decreases progressively,—at first gradually, then more rapidly, out to the limits of the group.
Naturally, the size of the cone of fire and of the shot group vary with the skill of those firing, good shots making a small cone and small group, and poor shots a large cone and large group.
[1413]. Beaten zone. The intersection of the cone of dispersion with the surface of the ground is called the beaten zone.
If the surface of the ground is horizontal, the form of the beaten zone is that of an ellipse with its longer axis in the direction of the line of fire, as shown in [Fig. 45].
Fig. 45
In view of the fact that at the long ranges the angle of fall of the bullets is much greater than at short ranges, it follows that at short ranges the elliptical figure (beaten zone) is much more elongated than at long ranges. In other words, the longer the range, the shorter is the depth of the beaten zone. This is shown in [Fig. 45].
[1414]. Uncertainty and ineffectiveness of long-range fire. It follows from what has been said, that as the range increases the length of the beaten zone decreases,—that is, a less depth of ground is held under fire. That being the case, if an error is made in sight setting due, for example, to an incorrect estimate of the range, the proportionate loss of fire effect due to misplacement of the center of impact will be greater as the beaten zone is less,—that is, as the range is greater.
Furthermore, the difficulty of exact range determination increases with the distance, the two influences combining to make long-range fire uncertain and usually ineffective.