2. When Collective Fire is Effective.—As already stated, collective fire will not as a rule produce results commensurate with the amount of ammunition expended, or fulfil the purpose for which it is used unless the target is included within the area beaten by 75 per cent. of the bullets directed upon it—that is to say, unless it lies in the zone of effective fire ([Fig. 7], C).
Fig. 7. Diagrams illustrating the dispersion in depth of concentrated collective fire at 1,500 yards and its distribution in depth by the use of combined sights.
3. When Collective Fire is not Effective.—If an error in sighting is made which would result in the nucleus of the cone of fire striking at a distance short of or beyond the target which is equal to half the depth measurement of the zone of effective fire, the target will not be included in this zone, and the fire will be ineffective ([Fig. 7], B).
4. When combined Sights are to be Used.—(i) Under service conditions it may be assumed that even if the range is measured with the range-finder, the probability of error in ranging and judging atmospheric influences, known as the error of the day, is such that, at distances beyond 1,000 yards, collective fire concentrated on any target with one sighting will probably be ineffective.
(ii) To give a satisfactory degree of assurance of fire effect, it is advisable in such cases, unless sighting can be corrected by observation of results, to distribute fire in depth by using two elevations differing by 100 yards. One of these elevations would be 50 yards over and the other 50 yards under the sighting believed to be correct.
(iii) There will thus be two cones of fire and two beaten zones overlapping between the nucleus of each cone of fire, and an even distribution of fire will be obtained over a zone about 150 yards deep ([Fig. 7], D). Thus, if one sighting had been used, as shown in [Fig. 7], B, the fire would have been ineffective, though the grouping of bullets in the zone of effective fire would have been more dense, because the error in sighting would have had the result described in para. 3 above. By using combined sights fire becomes effective, because though the grouping of bullets is less dense fire is distributed in depth so as to avoid the result described in para. 3.
(iv) Ordinarily fire should be closely concentrated with a view to observation of results, but if observation has failed, or if the situation calls for immediate application of effective fire, combined sights should be used at ranges beyond 1,000 yards. Combined sights may sometimes be used to increase assurance of effect when the enemy’s position is ill-defined, but should never be used if observation of results can be obtained. Combined sights should not be employed by bodies of less than two platoons.
Section 14.—The Relation of Ground to Fire Effect.
1. So important is the influence exercised on fire effect by the shape of the ground in relation to the grouping of bullets, that it is essential for all officers and non-commissioned officers to understand thoroughly how the probability of fire effect is increased or diminished by the inclination of ground with reference to the trajectory.