This shrapnel is used entirely against animate objects, its main purpose being to cover a given area with a powerful and effective bullet-fire, and must have, at the instant it bursts, a terminal velocity of 500 ft.-sec. to be effective. The trajectory of shrapnel until it bursts is identical with that of shell. On its bursting the bullets spread in every direction, and form a cone of more or less denseness. This cone will increase in size as the range increases, because the velocity of translation decreases more rapidly than the velocity of rotation. On the other hand, the area of the oval formed by the intersection of this cone with the ground will decrease as the angle of fall increases. This oval has its greatest depth in the direction of fire, and its broadest end furthest away from the gun. Its depth diminishes as the range increases, and for short ranges it is much greater than for long ones. The breadth increases as the distance short of the object at which the burst takes place increases. The nearest bullet (that is, the one which has travelled the shortest distance) will have the highest striking velocity, and the bullet that travels furthest will have the lowest striking velocity, the bullets between striking with proportionate velocities. As the distance short of the object at which the shell bursts increases so will the distances between the bullets on striking increase.
Fig. 107.
When firing at a horizontal target, the bullets are not uniformly spread over the whole of the oval of dispersion, those on the nearer side of the minor axis being more crowded together than those on the farther side. Of course the ricochets more or less endanger the omitted spaces, but as these are dependent on the hardness of the ground and on the angles that the strikes make with the actual surface of the ground with which they come in contact, they cannot be relied on. On favorable ground all the bullets will ricochet at all ranges, but the ricochets of those that fall short of the axial bullet will be about the only ones that will be effective.
The following rules should be observed:
1. When the object has depth, the fire cannot be too direct.
2. When the object has frontage and little depth, the more oblique the angle of fire is to the front the greater is the area affected.
3. Time-shrapnel is peculiarly adapted to objects moving toward or from the battery. In the first case the fuzes are set rather short, in the second rather long.
4. The best position generally for the point of burst is about six yards above and fifty yards in front of the target. A good rule for the height of burst is the height of burst in feet = the number of hundred yards in range. Thus for 1800 yards' range it would be 18 feet (6 yards).
5. The spread may in general be reckoned as ¼ to ⅓ the distance of burst.