Chapter II: The Theory and Practical Application of Rifle Fire

Section =8=.--General Information.

=1. Need of Theoretical Knowledge.=--A knowledge of the theory of rifle fire is of great importance in enabling the best use to be made of the powerful and accurate weapon with which the soldier is armed. The next step in the soldier’s musketry training after he has been instructed in the construction, care, and cleaning of his rifle will therefore be devoted to teaching him the theory and application of rifle fire. This instruction will consist of lectures, important points being illustrated by the aid of diagrams or drawings made with chalk on a blackboard, and by practical demonstrations when possible.

=2. Difference between Peace and War Conditions.=--(i) It is, however, of the utmost importance that the soldier should be taught from the first to apply his theoretical knowledge correctly. _The soldier must be made to realize clearly that the moral conditions under which fire is delivered in war are very different from those of peace._ Therefore deductions made as to the effect of fire from the theory of musketry applied under peace conditions, when firing takes place on open level ground under good conditions of light and atmosphere at visible targets at known distances with weapons in good condition in the total absence of any strain of excitement or fatigue and of the enemy’s fire, must not be applied to conclusions as to the effect of fire delivered under war conditions.

BM = _Axis of barrel_
MF = _Line of departure_
MS = _Line of fire_
LOS = _Line of sight_
MTS = _Trajectory_

=Fig. 2.= ILLUSTRATION OF TRAJECTORY ETC.]

AG = _Horizontal plane_
CMF = _Axis of barrel_
BMO = _Line of sight_
ML = ” ” _departure_
BMC = _Angle of tangent
elevation--T_
OMP = _Angle of sight--S_
FMP = _Angle of quadrant
elevation--QE_
LMP = _Angle of departure--D_
LMF = ” ” _jump--J_
MHO = _Trajectory_

=Fig. 3.= ILLUSTRATION OF TRAJECTORY ETC.]

(ii) For under war conditions targets beyond close range are usually invisible or indistinct, and frequently in motion; distances are uncertain and difficult to judge; firing takes place under all conditions of light and atmosphere over every variety of ground, under the strain of excitement and fatigue in the face of artillery and rifle fire, with weapons which rapidly become imperfect owing to heavy wear. Accordingly, the theory of rifle fire is considered in this book in relation to practical service conditions.

=3. Technical Terms.=--The different technical terms used in this chapter in explaining the theory and application of rifle fire will be found among the Definitions (p. xxii). Some of these terms are illustrated in Figs. 2 and 3. In addition, the following technical terms may conveniently be quoted in this paragraph:

(i) =Individual Fire.=--Individual fire is fire opened by individual soldiers without orders from a fire-unit commander. When men fire individually, each one selects his target, estimates the range, and regulates his fire according to his own judgment.

(ii) =Collective Fire.=--Collective fire is the fire of a number of men combined for a definite purpose under the orders of a fire-unit commander, who indicates targets, gives ranges, and controls fire as to rate, etc.

_Note._--Collective fire is necessarily controlled fire. _Individual fire will also be controlled if circumstances permit._

=4. Rifling.=--A gun-barrel is said to be rifled when it has spiral grooves cut down the bore. Rifling a barrel enables an elongated bullet to be used instead of the round “ball” of former days. The advantage of this form of bullet is that it has great weight in proportion to the surface directly opposed to the air. It has therefore great power of overcoming the resistance of the air, and thus maintaining its velocity and penetrating force[5] over greater distances than would otherwise be possible. Moreover, when the charge is fired, the bullet is forced into and follows the spiral grooves up the barrel, the effect of which causes it to leave the muzzle rotating or spinning on its longer axis. This spin tends to keep its point foremost, and therefore to ensure accuracy of flight.

=5. Forces Acting on the Bullet.=--Three forces act on the bullet: (_a_) The explosion of the charge, (_b_) gravity, and (_c_) the resistance of the air. The explosion of the charge drives the bullet forward. Gravity--the natural attraction which draws all unsupported bodies towards the centre of the earth with ever-increasing velocity--acts on the bullet immediately it leaves the muzzle. The resistance of the air causes the velocity of the bullet to decrease rapidly during its flight.

=6. Trajectory.=--The combined effect of these forces causes the bullet to travel in a curved line called the _trajectory_, the curvature of which becomes more pronounced the longer the bullet is exposed to their action. Thus, a Mark VI bullet leaving the muzzle of a service rifle at the rate of about 2,060 feet per second falls about 4½ inches below the line of departure in the first 100 yards. This drop is increased to about 20 inches at 200 yards. With Mark VII ammunition, giving a muzzle velocity of 2,440 feet per second, the drops at the above distances are about 3 inches and 13 inches respectively. Imaginary not actual curves of trajectories are shown in Figs. 6 and 8.

=7. Method of Explaining Trajectory.=[6]--(i) In explaining the trajectory to recruits it is not sufficient merely to show or draw a diagram representing a trajectory distorted in respect of height and range. When possible the actual path of the bullet through the air at various short ranges--say 400 and 800 yards--should be shown by means of discs raised on poles at every 100 yards, or some similar device.

(ii) It must be explained to the soldier that the further an object has to travel and the longer it is suspended in the air, the higher it must be thrown to counteract the force of gravity, because the greater the distance it travels and the longer it remains in the air the longer will the object be affected by the force of gravity. Consequently the longer the range the higher will the curve of the trajectory take the bullet in its flight, and the steeper will be the angle at which it will fall to the earth.

(iii) On the other hand, the shorter the distance an object is thrown, and the swifter its flight through the air, the less will it be influenced by the force of gravity during its flight, and the lower or flatter will its trajectory be in consequence. Thus, with the backsight adjusted for the distances in question, a Mark VI bullet does not rise above the height of a man on foot at 500 yards range, or above the height of a mounted man at 600 yards range. With Mark VII ammunition, the bullet does not rise above these heights at 600 and 700 yards range respectively.

=8. Firing at Close Range without Altering Sights.=--_It is therefore evident that effective fire can be maintained within close range without alteration of the backsight._ Apart from the flatness of the trajectory at close range, and the consequent inclusion of distances within close range in dangerous space (Sec. 9), there will seldom be opportunities for altering sights at close range on service, and necessary allowances for elevation must be made by aiming up and down (Sec. 21).

=9. Elevation.=--In order to allow for the fall of the bullet, owing to the force of gravity, it is necessary to direct,the line of departure as much above the object to be hit, as the bullet will fall below it at any given distance if the axis of the barrel of the rifle is pointed at the mark. This raising of the barrel to allow for the curve of the trajectory is termed _giving elevation_. The target must of necessity be kept in view. The rifle is therefore provided with sights, which permit the firer to give the elevation required whilst keeping his eye fixed on the mark.

=10. The Sighting of Rifles.=--(i) In the sighting of rifles a _mean graduation for each range has been adopted_, and a high general standard of accuracy for all practical purposes is thus obtained. Each rifle is carefully tested before issue, but it must be understood that _no two rifles behave in an exactly similar manner_, and that even if compensation could be made for every error in the sighting of the rifle before issue, the wear of parts and the loosening or tightening of screws, etc., would bring about faults from time to time which would affect the shooting of each rifle differently.

=11. Need of Knowledge of Each Weapon.=--_It is therefore necessary that every man should study the shooting of his own rifle_, and make himself acquainted with any incorrectness of the graduations marked on the backsight, in order that he may be in a position to give his rifle the correct elevation for the estimated or ascertained range of the target. _At longer ranges the backsight elevation may be regarded as the best possible guide to errors under all conditions, or any error may be ascertained by using a long-range sighting target._[7]

=12. Jump.=--Owing to the shock of discharge, a vibratory or wavy motion is set up in the barrel, which slightly affects the line of departure of the bullet as it leaves the muzzle, and which is known as “jump.” The causes and extent of jump are dealt with in the textbook of _Small Arms_. The effect of jump is allowed for in the sighting of the rifle, _and will not be taken into account by the soldier in aiming_.

=13. Drift.=--Drift is the term used to express the _lateral deviation_ of the bullet after it has left the barrel. This is due to the direction of the rifling, which causes the bullet to rotate from right over to the left in its flight, so that the point works over slightly to the left, owing to gyroscopic action. The consequent increased air-pressure on the right side of the bullet forces it to the left as it flies. _The deflection due to drift at distances within 1,000 yards is negligible, and need not be taken into account by the soldier in individual firing._ Beyond 1,000 yards up to the limit of effective rifle fire at about 1,400 yards, drift will carry the bullet about 7 feet to the left. This deflection, if necessary, will be taken into account by fire-unit commanders in directing concentrated fire at narrow-fronted targets at longer ranges.

=14. Effect of Fixing the Bayonet.=--(i) When the bayonet is fixed to the muzzle of the rifle, its weight checks the jump, and in consequence slightly affects the position of the muzzle at the moment of the departure of the shot, and the primary direction given to the bullet.

(ii) =Short Magazine Lee-Enfield Rifle.=--With Mark VI ammunition, the accuracy of the short magazine Lee-Enfield rifle is not appreciably affected by fixing the bayonet. With Mark VII ammunition, the soldier will aim down slightly in firing with fixed bayonet at ranges up to 600 yards.

(iii) =Lee-Metford or Charger-Loading Lee-Enfield Rifles.=--With Mark VII ammunition, fixing bayonets has a negligible effect. When firing Mark VI ammunition from the Lee-Metford or Charger-loading Lee-Enfield rifles with fixed bayonets at ranges up to 600 yards, the soldier should aim up very slightly, taking care, however, in doing so to aim low in all cases, and to aim at the ground-line in firing at troops advancing towards him.

=15. Effect of Resting the Rifle.=--For practical purposes, the shooting of the rifle is not affected by resting the muzzle or any portion of the stock lightly on earth or other substance.

=16. Effect of Oily Barrel.=--The first round fired from an oily barrel is liable to follow an erratic course, the rifle throwing sometimes high, sometimes low, and at other times to the right or left. A dry rag should therefore be passed through the bore before practice is commenced.

Section =9=.--Dangerous Space--Ricochets--Firing Up and Down Hill.

=1.= Dangerous space may broadly be defined as the whole ground covered by the trajectory of a bullet from the point where it could first come in contact with the top of an object fired at to the point where it falls to the earth (Figs. 4 and 5, and Figs. 6 and 8).

=2. Extent of Dangerous Space.=--The extent of dangerous space depends upon the following factors:

(i) =Range.=--Dangerous space decreases as the range increases, the reduction being due to the steeper angle at which the bullet descends at longer ranges (compare Figs. 4 and 5).

(ii) =Firer’s Position.=--The position of the firer and the consequent height of his rifle above the ground will affect dangerous space. _The nearer the rifle is to the ground when it is fired, the greater will be the extent of dangerous space._

(iii) =Height of Object.=--As a rule the higher the object fired at is, the greater will be the extent of dangerous space.

(iv) =Trajectory.=--The flatter the trajectory--the closer the bullet keeps to the ground in its flight--the greater as a rule will be the extent of dangerous space.

(v) =Conformation of Ground.=--The more nearly the slope of the ground conforms to the curve of the trajectory and to the fall of the bullet, the greater will be the extent of dangerous space (see Fig. 8).

=3. Example of Conditions affecting Dangerous Space.=--The following example illustrates the method by which the soldier may be made to realize the importance of this question, and the principle which underlies its influence upon the effect of fire: If he is firing in the _lying position_ with Mark VI ammunition, and aims at the ground-line of a prone figure at 500 yards range, the dangerous space of his fire will be _about 50 yards_. If, however, _he stands_ to fire at the same target, the dangerous space will be reduced to _about 40 yards_.

_Note._--The angles of descent are imaginary, and not accurate for any distance.]

=4. Ricochets.=--Bullets which rebound after striking the ground or any other obstacle, and continue their flight, are said to ricochet. Ricochets may occur from any surface, and bullets may ricochet two or even three times before their flight is finally arrested. At long range, they are less likely to ricochet from soft ground than from hard, smooth surfaces.

=5. Firing Up and Down Hill.=--(i) When a shot is fired at a target placed on the same level as the firer, the forces acting on the bullet cause it to travel in its greatest curve, and the greatest elevation for any given distance must therefore be given to the rifle. If a shot is fired perpendicularly upwards or downwards, no elevation is required, for the bullet will travel in an approximately straight line until its impetus is exhausted. Hence it follows that, when shooting up or down hill, less elevation is necessary than when the object is on the same level.

(ii) _For practical purposes, the effect of firing up and down all moderate slopes will be negligible, and may be disregarded by the soldier in taking aim on service._ Within close range, in firing up and down steep slopes, or slopes at an angle of 10 degrees and over, the decreased elevation necessary will usually be small, and may be allowed for by aiming down slightly. The correct elevation to be used in firing up or down hill at longer ranges in various circumstances is a matter for the judgment of fire-unit commanders, and can best be ascertained by careful observation of fire, when possible.

Section =10=.--Effects of Barometric Pressure, Temperature, Wind, and Light.

=1. Barometric Pressure and Temperature.=--Rifles are sighted for the following conditions: Barometric pressure, 30 inches (sea-level); thermometer, 60° F.; still air; a horizontal line of sight.

=2.= The rise and fall of barometric pressure and temperature affect the flight of the bullet and elevation by changing the density of the atmosphere, and so increasing or decreasing its resistance to the bullet. For practical purposes, the soldier may disregard the effects of barometric pressure and temperature as negligible. In operations conducted at considerable heights above sea-level, orders as to allowance for barometric pressure will, if necessary, be issued by the Staff.

=3. Effect of Wind=--(i) =Head Wind, or Wind from the Front.=--A head wind, or wind from the front, retards the bullet, and necessitates _more elevation_.

(ii) =Rear Wind.=--A rear wind lessens the resistance of the air, and necessitates _less elevation_.

(iii) =Side Wind.=--A side wind, or wind blowing _at right angles_ from either side across the front, acts on the greater surface of the bullet, and has, consequently, more influence on its flight than a wind blowing from the front or rear.

(iv) =Oblique Winds.=--Oblique winds, or winds blowing from any intermediate direction between a right angle and a front or rear wind, have the same effect in varying degree as side winds. Oblique winds, in addition, affect the bullet to some extent in the same way as head and rear winds.

=4. Allowance for Head and Rear Winds.=--No fixed rules can be laid down regarding the degree to which elevation should be increased or decreased at different ranges for head and rear winds. For practical purposes, the effect of these winds may be disregarded at ranges under 1,000 yards. At longer ranges, fire-unit commanders will use their judgment in directing fire, whether they will allow for strong head and rear winds by increasing or decreasing elevation, and to what extent.

=5. Allowance for Side and Oblique Winds= (see also Sec. 20, _Aiming-Off_).--(i) No fixed rules can be laid down as to the degree of allowance to be made by aiming-off for side and oblique winds of varying direction and velocity. Owing to the increased time during which the bullet is exposed to the effect of wind, and owing to the height attained in its flight, the allowance for wind at long ranges must be out of all proportion greater to that necessary at close range.

(ii) At close range, whether fire is distributed along the dense line of an infantry attack or concentrated on narrow-fronted attacking columns, the effect of side wind will cause little, if any, loss of fire effect. Individual soldiers, however, in taking the deliberate aim necessary for fire effect, should aim off for side winds of different velocity (see Sec. 20, para. 3).

(iii) In directing fire beyond close range, fire-unit commanders must use their judgment in allowing for deflection due to side and oblique winds by including directions for aiming-off in fire orders. The extent of the allowance will in every case depend upon a variety of factors, including the direction and velocity of the wind and the distance of the target. Neglect to make due allowance for strong winds at longer ranges may lead to loss of fire effect, more particularly in the case of concentrated fire aimed at narrow-fronted targets, such as a machine-gun.

=6. Effect of Light.=--In bad light, the foresight is less distinctly seen than in good light, and more of it is unconsciously taken into the line of sight. For practical purposes, the effect of light may be disregarded in firing at all ranges.

Section =11=.--Need for Collective Fire.

=1.= (i) Only exceptional targets and very favourable atmospheric conditions will justify soldiers in opening individual fire at distances beyond about 600 yards. Collective fire is necessary to give reasonable assurance of fire effect beyond about 600 yards up to about 1,400 yards, which is usually the limit of effective rifle fire. _Beyond 1,400 yards, the fire of even large and well-controlled units of infantry has seldom much effect upon the decision of the struggle for superiority of fire._

(ii) Collective concentrated fire is also used to form a cone of fire for observation of fire, when results can be noted by the dust raised by the strike of bullets on the ground or by the effect of fire upon the enemy. It must be clearly impressed upon the mind of the soldier at this stage that, _however skilful individual men may be as marksmen, the greatest effect is produced by their fire only when it is efficiently directed and controlled_.

=2.= (i) The soldier must also be made to understand clearly why effect cannot be obtained from individual fire in battle beyond close range, and why collective fire is necessary for fire effect at all ranges beyond close range. The answer to this question may be stated simply and very broadly as follows: In effect, for various reasons, the individual marksman cannot as a rule see his target clearly or at all with the naked eye beyond close range on service, and he can neither aim at it with accuracy nor be certain of its exact range. Moreover, the effect of inaccurate firing, whether due to excitement, fatigue, or other causes, increases with distance.

(ii) Clearly, therefore, the soldier cannot obtain reasonable assurance of fire effect by individual fire beyond close range. Accordingly, fire-unit commanders equipped with field-glasses discern and indicate targets to him. They give him ranges after they have been ascertained by the use of range-finding instruments, or by observation of fire, when possible, and they direct the fire of a number of individual soldiers in a volume which, whether distributed or concentrated, is sufficient to compensate for individual errors, and so obtain fire effect as described in Secs. 12 and 13, and in Chapter VI.

=3.= The principal reasons which militate against the assurance of fire effect, by individual fire, beyond close range may be summed up as follows: The effects of fatigue and excitement on the firer; the effects of atmosphere, heat, and light; errors due to imperfections in the rifle and ammunition; uncertainty in estimating longer ranges; the difficulty of discerning and aiming at small, moving, and indistinct or invisible targets; and the steepness of the angle of descent of the bullet, which becomes rapidly accentuated, and so decreases dangerous space as range increases.

=4.= The soldier must be made to realize that the results of errors of marksmanship due to these various causes _increase in extent as the range increases_, and that they reach their maximum at longer ranges; that is to say, targets become smaller and more difficult to see and to aim at, their range becomes harder to estimate, and the accuracy of fire for other reasons stated becomes less as the range increases. _Therefore, the effect of individual fire decreases as the range increases._ It is easily possible for instructors in the course of lectures on this point to make it sufficiently clear to men that, under service conditions at longer ranges, effect must be obtained from collective fire rather than from individual fire, for all the above reasons. In some degree this important principle of the science of modern warfare can be demonstrated by firing at service targets on full distance and also on miniature ranges (see Chapter X).

Section =12.=--Dispersion of Individual and Collective Fire.

=1. Shot Groups.=--Owing to errors on the part of the firer, and also to imperfections in the rifle and ammunition, it is found that a series of shots, even when fired by an individual under perfect conditions at a large, stationary, distinct target at a known and close range, do not all strike the point aimed at, but they form a group of shot-marks about this point, the density of which varies mainly with the skill of the firer.

=2. Cone of Fire= (Fig. 6).--It is evident that the trajectories of these shots will not coincide, but will together form a figure termed “the cone of fire.” It is also clear that, when aim is well directed, the bullets of a cone of fire should always strike an object so long as the shot group formed by it is either _smaller or of the same size_ as the object aimed at. But when the shot group is spread over a larger surface than the size of the object, shots must necessarily miss the mark in proportion as the size of the grouping exceeds that of the object aimed at.

A--B shows the dangerous space due to height of objective.

_Note:-The Dangerous Space will vary with height of objective.
The curve of the Trajectory is imaginary and not accurate for any
distance._]

=3. Dispersion of Individual Fire.=--It is clear, therefore, that the dispersion of individual fire explained in para. I will of itself, apart from the various factors mentioned in Sec. II, militate against the assurance of fire effect by individual fire at longer ranges, because the dispersion of fire will become greater as the target decreases in size, owing to distance, and the shot group will consequently be spread over a much larger surface than that of a long-range service target, even if stationary and visible to the naked eye.

=4. Dispersion of Collective Fire.=--When a body of soldiers fires with the same elevation at the same object, the dispersion of shots is accentuated by the varying skill and eyesight of the men, with the result that the cone formed will be of larger dimensions than in the case of individual fire. The dimensions of the cone of fire will again be further increased if the firers are from any cause, such as fatigue or excitement, unsteady; if the rifles are in bad condition or the target indistinctly seen--factors which are all incidental to firing under the conditions of active service.

=5. The Beaten Zone.=--(i) The area of ground beaten by a cone of fire is termed the “beaten zone.” It is regarded as a plane surface. Fig. 6 illustrates this zone together with the additional space which would have to be taken into account in reckoning the dangerous space of the cone of fire in consequence of the height of the objective.

(ii) =Depth of Beaten Zone.=--The depth of the beaten zone at short ranges need not be considered on account of the flatness of the trajectory, which insures that practically its whole extent from the firers to the target is swept by bullets. The depth of the beaten zone at various ranges with the Mark VI and Mark VII ammunition is as follows:

+-------------+-----------------------+
| Distance. | Depth of Beaten Zone. |
| +-----------+-----------+
| | Mark VI. | Mark VII. |
+-------------+-----------+-----------+
| 500 yards | 220 yards | 300 yards |
| 1,000 ” | 120 ” | 180 ” |
| 1,500 ” | 100 ” | 120 ” |
+-------------+-----------+-----------+

(iii) It will be seen that up to 1,500 yards--when the ground is parallel to the line of sight--the depth of the beaten zone decreases with the range, on account of the increased angle of descent of the bullets. This shrinkage of the beaten zone does not of itself increase fire effect by the closer grouping of bullets, owing to the increased steepness of the angle of descent, which decreases dangerous space.

(iv) Beyond 1,500 yards it will be found that the depth of the beaten zone tends to increase in consequence of the influence of atmospheric conditions on the flight of the bullet, the increased effects of errors in aiming, and faults in the rifle and ammunition--all of which combine to increase the dispersion of fire. This increased dispersion does not of itself increase fire effect owing again to the greatly increased steepness of the angle of descent at longer ranges, which results in a correspondingly great decrease in dangerous space.

(v) =Lateral Dispersion.=--On the other hand, owing to faults in aiming, comparative invisibility of the target, inaccuracies in the rifle and ammunition, and atmospheric influences, the _lateral dispersion_ of a cone of fire _increases as the range becomes greater_. With Mark VI ammunition, the dispersion of the best 75 per cent. of shots fired may be taken as 7 feet by 220 yards at 500 yards, 14 feet by 120 yards at 1,000 yards, and 22 feet by 100 yards at 1,500 yards.

=6. Nucleus of Cone of Fire.=--It is found that the bullets in a cone of fire _are not dispersed evenly over the surface of the beaten zone_. It is found that they are grouped in such a way _that the majority of the bullets fall in the general direction of the line of fire, that the density of the grouping decreases progressively from the centre to the extreme limits of the beaten zone, and that the bullets are collected most thickly near the point for which the sights were set_. This dense grouping is usually termed the nucleus of the cone of fire, and is regarded for purposes of comparison as including the best 50 shots per cent.

=7. Zone of Effective Fire.=--The area of ground beaten by the best 75 shots per cent. fired is termed the zone of effective fire, for it has been found by experiment that _useful results in battle can only be looked for when the target is within these limits_.

Section =13=.--Searching.

=1. Definition of Searching.=--Searching is the term applied to collective fire when the depth of its dispersion over a beaten zone is increased by the use of combined sights as described in para. 4 of this section.

=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).

A B C D

Nucleus Error Target
on exceeding half included Combined
Target depth of in Sights
Effective Zone Effective Zone

=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.

=2.= In attack, such knowledge will assist them in _adopting formations and directing the fire of their men to the best advantage_. In defence, it will aid them _to select the best positions for fire action_, and will enable them to take steps to minimize the inherent disadvantages of these positions. In the following examples only _plane surfaces, without undulations or accidents, are considered_.

=3. Level Ground.=--As already stated, on level ground the zone beaten by collective rifle fire varies considerably with the range. The extent of this zone is further influenced by the _inclination of the ground to the line of sight_.

=4. Rising Ground.=--(i) The more steeply the ground rises with reference to the line of sight, the greater will be the decrease in the extent of the beaten zone. For example, when firing with Mark VI ammunition at ground rising 2, 5, and 10 degrees, the depth of the beaten zone at 1,500 yards range is decreased roughly by quarter, half, and two-thirds respectively (Fig. 8). Since the grouping of the bullets becomes closer as the upward slope of the ground increases, the effect of errors in estimation of range will be more serious, and as the bullets will fall at a steep angle the dangerous space will be proportionately reduced.

_The Beaten zone is greatest on slope A-D, the Trajectory being
practically parallel to the surface of the ground._

=Fig. 8.= THE RELATION OF GROUND TO FIRE EFFECT.]

(ii) =Formations, Supports, and Reserves.=--As a general rule, therefore, on ground which rises with reference to the line of sight, _troops should be drawn up in shallow formations, but supports and reserves may be nearer the firing-line than is normally advisable_.

=5. Falling Ground.=--(i) When the ground beaten by bullets falls in respect of the line of sight, _the depth of the beaten zone is augmented in proportion as the downward slope increases, until it reaches its greatest magnitude when the angle of the fall of the bullets is the same as the slope of the ground_; or, in other words, when the trajectory is practically parallel to the ground surface (Fig. 8, Depth of Zone, A--D). In these circumstances the fire becomes _grazing_ (Fig. 9), _and the extent of the dangerous space is nearly identical with the beaten zone_.

(ii) _At close ranges_, therefore, where the trajectories are flat, the depth of the beaten zone will be much increased if the ground behind the target _falls at a gentle slope_ (Fig. 9, A). _At long ranges_, on the other hand, a greater area will be beaten when ground falls more steeply (Fig. 9, C).

(iii) For example, at 1,500 yards range the depth of the beaten zone is roughly increased by three-fifths when the bullets strike ground falling at 2 degrees. On ground falling at 5 degrees, which is nearly parallel to the trajectory at this range, the depth beaten is about ten times greater than on level ground.

(iv) It is clear, therefore, that falling ground _far behind the objective will at times be swept by unaimed fire_, and it follows that in such circumstances supports should either be under cover or, if there is no cover, in shallow columns on narrow frontages with the object of reducing target surface as much as possible.

=6. Crest Line.=--(i) When the objective is a crest line the depth of the beaten zone is greatest, and part of the fire is grazing, when the ground beyond the crest is parallel, or nearly so, to the trajectory of the bullets (Fig. 9, A and B). At close ranges, in this case, there will be behind the crest-line a _defiladed zone, or space not swept by fire_ (Fig. 9, C), greater or less according to the distance from which fire is delivered, the inclination of the line of sight, the extent of the hill top, and the inclination of the reverse slope.

(ii) =Position of Supports and Reserve.=--It appears, therefore, _that when the firing-line is placed on the crest of a razor-backed hill_, with steep reverse slopes (Fig. 9, C), supports and reserves will at all ranges be but little exposed to unaimed fire when posted in its vicinity. In other cases, when the crest of a hill is occupied, the vulnerability of supports and reserves will be least if, when the enemy is at long range, they are withdrawn from, and as he approached closed on, the firing-line (Fig. 9, C).

=7. Dead Ground= (Fig. 10).--Dead ground is ground on which, owing to its conformation or to the existence of natural or artificial cover, fire cannot be brought to bear from a given locality. The term is thus a relative one, for though an area of ground may be dead in relation to one locality it may be possible to bring fire to bear upon it from another locality, in relation to which it will not therefore be dead.