Chapter IV: Gunnery

=96.= _General instructions._

1. The subjects dealt with in this chapter are only divided into sections for purposes of reference. The contents of many of the sections are so closely connected that portions of several of them must often be included in one lecture. In addition it must be realized that it is impossible to deal fully within the limits of a manual with the various subjects mentioned. All that is attempted is to give a brief outline of the principles of modern gunnery.

2. A description of the various patterns of ordnance used in the field, their stores and drill will be found in the respective handbooks; while for further information of a more technical nature reference should be made to the “Text Book of Gunnery,” the “Treatise on Ammunition,” and the “Treatise on Service Explosives.” Instructors should endeavour to use the simplest language; but, as a large number of more or less technical terms must be employed, they must take care that their meaning is clear.

=97.= _Gunnery terms._

_Angle of Departure._--The angle which the line of departure makes with the horizontal plane, in other words, the quadrant angle plus the jump. (_See_ 5, Plate II.)

_Angle of Descent._--The angle which the trajectory makes with the line of sight at the point of their second intersection.

_Angle of Incidence._--The angle which the trajectory makes with the normal to the surface struck.

_Angle of Elevation._--The angle which the line of sight makes with the axis of the gun. (_See_ 2, Plate II.)

_Angle of Sight._--The angle which the line of sight makes with the horizontal plane. (_See_ 3, Plate II.)

_Axis of the Gun._--A line passing down the centre of the bore. (_See_ A B, Plate I.)

_Axis of the Trunnions._--A line passing through the centre of the trunnions. (_See_ C D, Plate I.)

_Battery Angle._--The angle formed at the battery by imaginary lines drawn to the target and the observing station.

_Calibre._--The diameter of the bore in inches measured across the lands.

_Drift._--The constant deflection of the shell due to the rotation imparted by the rifling. (_See_ A, Plate III.)

_Direct Laying._--When the gun is laid by looking over or through the sights at the target.

_Indirect Laying._--When the gun is laid for direction on an aiming point, or on aiming posts, the angle of sight is adjusted by clinometer and the elevation by the range indicator or drum. (With the 5“ B.L. howitzer the range drum is set at the quadrant elevation.)

_Jump._--The angle between the line of departure and the axis of the piece before firing. It is due to the vertical movement of the gun on firing, and for any gun differs according to the mounting and the charge used. (_See_ 6, Plate II.)

_Lateral Deviation._--The distance of the point of impact of the projectile right or left of the line of fire.

_Line of Departure._--The direction of the shell on leaving the muzzle. (_See_ 4, Plate II.)

_Line of Fire._--A line joining the muzzle of the piece and the target.

_Line of Sight._--A straight line passing through the sights and the point aimed at. (_See_ E F, Plate III.)

_Muzzle Velocity_--The velocity in feet per second with which a shell leaves the muzzle.

_Point Blank._--A gun is laid point blank when the line of sight is parallel to its axis.

_Quadrant Angle._--The angle which the axis of the piece makes with the horizontal plane. It is termed quadrant elevation or depression according as the gun is laid above or below the horizontal plane. (_See_ 1, Plate II.) The angle of elevation and the quadrant angle are the same when the line of sight is horizontal.

_Range._--The distance to the second intersection of the trajectory with the line of sight.

_Ranging._--Ranging is the process of finding the elevation, fuze, and line.

_Remaining Velocity._--The velocity of a shell at any given point of its trajectory.

_Striking Velocity._--The velocity of a shell at the point of impact.

_Trajectory._--The curve described by the shell in its flight. (_See_ D F, Plate III.)

=98.= _Natures of artillery fire._

(_See_ Fig. 11.)

_High Angle Fire._--Fire from guns and howitzers at all angles of elevation exceeding 25 degrees. But in coast defence the term “High Angle Fire” is used in connection with guns of mountings specially designed for extreme angles of elevation laid by means of instruments specially provided.

_Enfilade Fire._--Fire which sweeps a line of troops or defences from a flank.

_Frontal Fire._--When the line of fire is perpendicular to the front of the target.

_Oblique Fire._--When the line of fire is inclined to the front of the target.

_Reverse Fire._--When the rear instead of the front of the target is fired at.

=99.= _Gunnery._

1. Gunnery is the science of directing a projectile so that it will strike a given object.

2. A gun serves two purposes. First to confine the gases of the charge so as to allow them to act upon the base of the shell; and second to give the shell the proper initial direction. In order to maintain the shell in its proper direction after it leaves the bore of the gun, some means of making it spin or rotate rapidly is necessary; since it is a well-known fact that any rapidly-rotating body tries to keep the same direction in which it was pointed when first made to spin. Rifling which consists in a number of grooves cut down the bore, leaving raised ribs called “_lands_” between them, is the means employed in modern guns, in combination with a soft copper band called a “_driving band_” secured to the shell near its base. The result of this combination is that when the gun is fired the shell is forced along the bore, but, the diameter of the driving band being bigger than that of the bore, the lands cut into the copper of which it is made and the shell is consequently compelled to follow their course and rotate. Without this spin an elongated shell would soon lose its velocity and accuracy of direction and become as unreliable in its flight as round shot were in the days of smooth bore guns.

3. By the use of elongated shell the following additional advantages besides accuracy of direction are obtained:--

i. A longer and therefore heavier shell can be fired from a
field gun than would be possible if the shell had to be
round (spherical). Consequently there is more room in
the shell for explosive or bullets.
ii. Greater range and greater power at a given range are
obtained, because there is a smaller surface, as
compared with a round shot of the same weight, offered
to the resistance of the air.
iii. By varying the length different kinds of shell for
the same gun can be brought to the same weight and
thus complications in range tables can be avoided. If
necessary a specially heavy projectile can be used.
iv. The flight of the shell being regular, allowance can
be made for any deviation observed and thus increased
accuracy may be obtained.

=100.= _Rifling._

1. The “_system of rifling_” is the term applied to the method adopted in any particular type of rifled gun for giving rotation to the shell.

2. The aim of each system is to produce accuracy of fire, but it is also essential that it should be simple, that it should not seriously weaken the durability of the gun, and that the shell should not be liable to jam in loading or firing. The “_twist of rifling_” by which is meant the distance measured in calibres, in which the grooves make one complete circuit of the bore, may be uniform, increasing or a combination of the two.

3. In the case of a uniform twist the distance in which the grooves make a complete circuit of the bore is the same wherever measured, but with an increasing twist the distance decreases as the muzzle is approached.

With a uniform twist the shell is compelled to rotate rapidly as soon as it begins to move, and thus a severe strain is caused both to the gun and shell.

With an increasing twist, this rotation is imparted more gradually, thus relieving the strain, but at the same time causing more friction, and consequent loss of velocity. The shell, moreover, is not so well centred as with a uniform twist.

4. In designing a gun the twist has to be made to suit the shell which it is desired to employ, and its intended velocity. Generally speaking, a long shell with low velocity necessitates a rapid twist to make it steady in flight.

=101=. _Centring._

It is important that the shell when it leaves the bore should be centred, _i.e._, that the shell should rotate round its longer axis which should coincide with the prolongation of the axis of the gun at the moment it leaves the bore. Should this not be the case, the shell becomes unsteady and noisy in its flight, and the shooting will be irregular.

=102.= _Forces acting on a shell in the bore._

1. The velocity attained by a shell at the muzzle of a gun is due to the pressure of the gas during its passage through the bore. The more gradually this velocity is imparted to the shell the less will be the strain upon it and the gun. The object sought is to distribute, as far as possible, the pressure over the whole length of the bore and to obtain the maximum work from a given charge without undue strain on either gun or shell. Theoretically the last atom of the charge should be converted into gas as the shell leaves the muzzle.

FORCES ACTING ON A SHELL DURING FLIGHT.

=103.= _The resistance of the air._

1. The air consists of innumerable small particles through which a shell has to force its way. This produces a rapid loss of velocity; for instance, the velocity of a shell from the 18-pr. Q.F. gun, which at the muzzle is about 1,610 feet a second, is at 2,000 yards only about 1,030 feet a second, and at 6,000 yards about 740 feet a second.

2. The retardation due to the resistance of the air varies according to the weight and diameter of the shell. If, for instance, two shells of equal diameter, but of different weights, start with the same muzzle velocity, the heavier will lose its velocity more slowly and have the longer range, because it has the greater weight with which to overcome the resistance of the air. On the other hand, if two projectiles are of the same weight but of different diameters, the one with the smaller diameter will have the advantage, because it presents less surface to the resistance of the air.

3. The shape of the head also materially affects the question, for a shell with a blunt head is plainly not so suited for forcing its way through the air as one with a more pointed head. Thus the longer the shell (other things being equal), the greater will be its remaining velocity at any given range, and the greater will be its range for any given muzzle velocity; but other considerations limit its length, such as the strength of its walls, and the liability to turn over in flight.

4. The relationship between the weight and diameter of a shell, represented by the expression

W
------,
(n d²)

is called the “_ballistic coefficient_” of the gun. The factor n includes sub-factors which vary with the density of the atmosphere, the shape of the head, the steadiness in flight, &c. The greater the ballistic coefficient the less rapidly does the projectile lose its velocity.

5. It has already been pointed out (Sec. =99=) that spin is necessary to the shell if elongated projectiles are to be used. The effect of the resistance of the air on a spinning shell is to deflect it in the same direction as the spin. With all service shell this spin or rotation is right handed, and consequently all shell deflect towards the right in the course of their flight. This deflection is called “_drift_.”

As this rotation is definite and diminishes very slightly during the flight of the shell, the amount of drift can be determined for each nature of gun by actual experiment and compensation made for it. A description of the method by which this is done will be found in Sec. 119.

=104=. _The force of gravity._

1. The force of gravity is the natural attraction which causes every unsupported body to fall towards the centre of the earth. Moreover, the longer a body is exposed to its influence the faster does it fall; thus a body falls,

About 16 feet in the 1st second
” 48 ” ” 2nd ”
” 80 ” ” 3rd ”

that is, the total drop at the end of any given second of time is proportionate to the square of the time.

Now a shell leaves the muzzle of a gun with a certain velocity, due to the forces which acted on it in the bore, and, if the effect of these forces and of the resistance of the air had alone to be taken into account, it would proceed in a straight line, but at a decreasing pace.

Gravity, however, comes into play and causes the shell to fall with a constantly increasing velocity. Thus in Fig. 12, supposing AB is the direction in which a shell starts, the distances it would travel in the first three seconds of its flight, if there were no gravity, might be represented by AC, CE, EG. Owing to gravity, however, it would at the end of the 1st second have dropped to some point D; at the end of the 2nd second it would have dropped four times as far, to a point F; while at the end of the 3rd second it would have fallen nine times as far, to a point H.

2. The force of gravity is to some extent counterbalanced by the air, which acts as a cushion and tends to support a body falling through it. In the case of a heavy body, such as a bullet or shell, the distances fallen are but slightly affected by the air.

Gravity is, however, appreciably counterbalanced by the “_planeing_” effect on an elongated projectile forced at considerable velocity through the air at high angles of elevation so that the value of “g” drops below 32 to about 26.

=105.= _Trajectory._

1. The result of the three forces acting on the shell--the force of projection tending to drive it forward in a straight line in prolongation of the axis, the force of gravity drawing it down from that line, and the resistance of the air tending to stop its progress in each successive instant of time, and also to deflect it to the right--is that it describes a curve, whether regarded from the side or from above. This curve is called the trajectory. (_See_ ADFHI, Fig. 12.)

2. The shell in consequence of its high velocity, and of the short time gravity has had to act upon it, falls at first very little, but this fall--and consequently the curve--increases very rapidly with the range; for example, at 100 yards the 18-pr. Q.F. shell if fired point blank would fall about 8½ inches, but at 500 yards its drop would be about 18¼ feet; at 1,000 yards, about 80 feet; at 2,000 yards, about 372 feet; at 3,000 yards, about 970 feet. The greatest height attained by the trajectory is roughly four times the square of the time of flight.

=106.= _Elevation._

1. It is evident from what has been said that a shell fired from a gun at a mark, S (Fig. 13) will not hit that mark, but will strike some point, D, below it. To allow for the fall, it is necessary to point the axis as much above the object to be hit as the shell would have fallen below it if the axis had been pointed straight at the mark. This act of tilting the gun so as to allow for the curve of the trajectory, is what is meant by the expression “_giving elevation_.”

2. The theory of giving elevation may be illustrated by reference to the flight of an 18-pr. Q.F. shell for the first 100 yards. This shell falls about 8½ inches below the line at which the projectile leaves the bore in passing over the first 100 yards of its flight. Assuming that S (Fig. 13) is 100 yards from the muzzle and that SD represents 8½ inches, the axis of the gun must be elevated so that, when produced, it would pass 8½ inches above S, viz., through F (Fig. 14), if an object at S is to be hit.

3. The method of giving elevation to the gun to enable the shell to reach a certain distance called the “_range_” should then be explained, and it should be shown how the elevation can be marked in yards on the range drum, tangent sight or clinometer.

4. The principle of the sighting can be shown by means of a rifle barrel placed on a tripod a few yards from a blackboard. If then aim be taken at some mark on the board with the leaf of the sight upright, and set to (say) 1,000 yards; and subsequently, without moving the barrel, the bore be looked through, it will be seen that the axis produced meets the blackboard considerably above the mark aimed at.

5. The effect on the shooting of the gun, if the wheels are not level, can be explained by similar means. A stick to represent the trunnions of the gun should be fastened across the barrel. It can then be shown that if the wheels of a gun are not level, or in other words if the axis of the trunnions is not horizontal, the gun (barrel) will not elevate in a vertical plane; the muzzle will move towards the lower side and the breech towards the higher, thus causing the shell to move towards the lower wheel.

6. The effect on the sight, if the axis of the trunnions is not level, can also be demonstrated. The more the sight is tilted over to one side the less will the elevation become and the more will the gun point to the right or left of the target, according to the way the sight is inclined. In these circumstances the shell, instead of hitting the target, will fall below it and to one side. The greater the elevation on the sight and the more it is inclined, the greater will be the resulting error.

To make this clear, a vertical line, AB (Fig. 15), may be drawn on the blackboard, with A as a spot to aim at. The recruit should then be directed to aim at this spot with the sight, which must be perfectly upright, set at 1,000 yards, and afterwards to look through the rifle barrel, his attention being drawn to the fact that the axis cuts the vertical line at C above the spot aimed at. He should then be made to aim at the same spot with the same sight, _but inclined to one side_, and to look through the barrel again, when he will see that the axis, instead of being directed upon C as before, is now directed low, and to that side to which the sight is inclined, as at D; consequently as the trajectory always conforms to the movement of the axis, the bullet, instead of hitting the mark, would strike as much below D as A is below C. Draw a new vertical line through D and measure off on it a distance equal to CA; this will give the spot E, which the bullet would hit. From A draw a horizontal line AF to the new vertical line DF, then AF will show the error of direction, and FE the loss of elevation. The latter may practically be neglected; and a rule is given in Sec. =119= for correcting the former, when the small size of the target renders it necessary.

If it can be arranged it is better to show this with a gun, but a rifle barrel is usually more convenient.

=107.= _Causes affecting the accuracy of shooting._

1. With the exception of the effect of the difference of level of wheels, the various forces which act on a projectile and their effects on its flight are calculated and compensated for before the gun is issued for service.

They will not, therefore, come under the observation of the practical gunner; but there are other causes affecting the accuracy of shooting about which he must have a knowledge.

These are:--

i. Varying effect of the charge due to:--
Incorrect weighing.
Variation in strength.
State of the atmosphere.
Variation in space occupied by the cartridge
in the bore (_Loading density_).
ii. Force and direction of wind.
iii. Trail not being well supported.

2. If the shell is not rammed home to the same spot each round, the amount of space available for the cartridge will vary and this will cause the shooting to be irregular. The smaller the space left for the cartridge the greater will be the range, but the greater also becomes the pressure of the gas in the chamber.

The possibility of this occurring in the case of Q.F. guns is prevented if fixed ammunition is used.

3. Wind has considerable effect on the range and direction of the shell. According to its direction it may increase or reduce the range, or drive the shell to right or left. If gusty, and of great force, the shooting will be irregular, especially at long ranges. This becomes of importance only when the target is a narrow one.

4. Unless the trail rests on firm level ground, and is similarly supported during successive rounds, the shooting will be irregular, on account of the variation of the jump. A carriage allowing of axial recoil has practically no jump.

5. It is important that the effect which the above conditions may have on the shooting should be fully understood, in order that the necessity of the process called “ranging”[6] may be realized. By means of this process the total effect of these causes is found out and the sight set so as to counterbalance them. (_See_ Sec. =103=.)

[6] “Ranging” and “Range-finding” must not be confused with one another. The latter is the measurement of the distance in yards to the target by mechanical means, and the former the process by which the elevation necessary to make a shell travel that distance is ascertained, an elevation which, owing to the various causes mentioned above, will rarely correspond with the distance given by the range-finder.

AMMUNITION.

=108.= _Cordite._

(_See also_ Treatise on Service Explosives.)

1. Cordite is the smokeless explosive adopted for propulsive purposes.

2. Cordite consists of nitro-glycerine, gun cotton and mineral jelly, worked into a dough and forced through a die, from which it comes out in long cords. Its composition for small arms and all natures of ordnance is the same, the required rate of combustion being obtained by varying the diameter of the cord. The smaller the diameter the quicker the rate of burning.

3. The cordite in general use for modern guns is termed “Cordite MD” (modified) when in sticks or cords, “Cordite MDT” when in tubes, in order to distinguish it from the earlier pattern of cordite which was found to wear out the rifling of the gun too quickly. Cordite Mk. I is still used in India and for the older patterns of guns and howitzers.

4. Tubular cordite is distinguished by two numbers representing in hundredths of an inch the mean external and internal diameters of the finished article. Other cordite is distinguished by a number representing in hundredths of an inch the diameter of the die through which it was pressed in the course of manufacture.

5. When any particular size is made to more than one length, the length will be included, _e.g._:--

Cordite, Size--, length 12-inch.
Cordite, Size--, on drums.

6. Cordite will keep in all climates, but it is laid down that the temperature of magazines in which cordite is stored should not habitually exceed 80° F. or fall below 45° F.

The nitro-glycerine in cordite freezes at about 40° F., and if frozen cordite is suddenly thawed it is liable to “sweat.” It should not be handled till the nitro-glycerine has been reabsorbed.

7. The ballistic properties of cordite are affected by heat; the higher the temperature of a charge the greater the muzzle velocity and pressure.

8. Cordite charges are somewhat difficult to ignite, so that a priming of guncotton yarn, fine grain gunpowder, or some other easily-lighted substance is necessary to assist the flame from the cap or tube.

9. Cordite is safe to handle and store. It is not affected by damp or water; this does not apply, of course, to the gunpowder priming. If wetted with fresh, water a cordite charge may be fired when dried. Before returning a wetted charge to store it should be thoroughly dried in a ventilated building. Cordite wetted with sea water should be well washed in fresh water and dried before repacking.

10. In some cases, after firing cordite charges to windward, a flame issues from the breech when it is opened. This is not of sufficient importance to require, in the case of field guns, any special precautions being taken.

=109.= _Lyddite shell._

1. Lyddite is the high explosive used for filling common shells.

It consists of picric acid, melted and poured into the shell, where it solidifies. In order to detonate the charge the shell are primed with exploders containing picric powder. Complete detonation of a lyddite shell may be assumed to have occurred when the smoke is black and not tinged with yellow, when there is no picric acid colouring the crater formed by the burst, and when the fragments of the forged steel shell present a torn and jagged appearance.

2. Shell filled with lyddite are effective against _matériel_ and artificial cover. They are not intended nor are they suitable for use against personnel in the open owing to the limited numbers of pieces into which they break up and to the very local effect caused by their explosion.

Lyddite shell are carried with howitzer, heavy batteries and mountain batteries armed with 2·75“ B.L. equipment.

=110.= _Shrapnel shell._

1. Shrapnel shell are hollow shell containing as many bullets as possible together with a bursting charge sufficient to open the shell, release the bullets, and give enough smoke to allow the burst to be observed. They are provided with T. and P. fuzes, thus making it possible to burst them, either on graze (percussion action) or at some selected point of their trajectory (time action).

In the latter case each bullet as soon as it is free follows a trajectory of its own, according to its position in the shell, to the direction and velocity given to it by the bursting charge and by the centrifugal force imparted by the rotation of the shell. Regarded as a whole, however, the bullets form a cone the apex of which is located at the spot where the shell bursts. This cone is called the “_cone of dispersion_.”

2. If the height of burst is normal, the body of the shell which does not as a rule break up has a trajectory which corresponds approximately to, but is always lower than, that which the shell would have described if it had not burst.

3. The effect of an individual shrapnel depends upon:--

i. The number of bullets.
ii. The energy of the bullets on impact.
iii. The spread of the bullets.
iv. The size of the target.
v. The curve of the trajectory, or the angle of descent.

4. The number of the bullets is very closely connected with the weight and construction of the shrapnel, and also depends upon the weight and specific gravity of the bullets.

The greater the weight of the shell, the greater under similar conditions will naturally be the weight and also the number of the bullets. The weight of the total content of bullets increases, however, more rapidly than the weight of the shell, so that of two shells constructed on the same principles, the heavier contains more bullets in proportion to its weight than the lighter. Thus for example, the 13-pr. Q.F. shell which has a weight of 12½ pounds contains 236 bullets of 41 to the pound, while the 18-pr. Q.F. has a weight of 18½ pounds and contains 375 bullets.

5. A very small energy is required to put living targets out of action. According to experiments it is considered that a striking energy of 60 foot-lbs. is sufficient; which means that a bullet of 41 to the pound would require a striking velocity of about 400 foot-seconds.[7]

[7] NOTE.--This is obtained from the formula

v2
-------
w × 2g

representing the energy of a body. In this case

v2
------- = 60; whence v = 400 (approx.).
41 × 2g

The velocity on impact of shrapnel bullets depends upon their velocity at the point of burst, their weight and upon the distance they travel after burst. The latter is of the more importance, since the light shrapnel bullets fall off very much in velocity owing to the resistance of the air.

6. In shrapnel shell with base bursters the spread of the bullets is principally caused by the rotation of the shell. In the 13-pr. and 18-pr. shell, however, the central tube is filled with powder, which tends to increase the angle of opening.

It is difficult to measure this angle accurately; but its tendency is to increase as the range increases. In estimating the front covered by the spread of the bullets, it may be taken as about 35 per cent. of the distance burst short.

7. The effect of the shrapnel depends largely on the nature of the target and the position of the burst. As the distance of the burst short of the target increases, the density of the hits diminishes, and theoretically this distance should be regulated according to the surface presented by the target. The normal height of burst of 13 and 18-pr. Q.F. shrapnel is about 10 minutes above the line of sight at all ranges. In the case of the 15-pr. B.L.C. and 15-pr. Q.F. this height is 12½ minutes, and in the case of 4·5-inch howitzers about 20 minutes, of 5-inch howitzers 30 minutes, and in the case of the 60-pr. B.L. and 4·7-inch Q.F. about 15 minutes. If the target be of the nature of a column, a lower burst must be obtained.

8. The curve of the trajectory diminishes the depth of the forward effect of the shrapnel. The flatter the trajectory the greater the depth of effect. On the other hand there is little searching effect on troops behind cover, and for this reason the shrapnel of howitzers fired at high angles of elevation are particularly effective, although the ground covered by their cone of dispersion is small.

9. Shrapnel shell is the principal field artillery projectile, and is carried for most natures of field artillery.

10. _Percussion shrapnel._--Percussion shrapnel is used for ranging, and in the case of the 18-pr. Q.F. has given excellent results at targets placed behind a brick wall 24 inches thick. It may therefore be considered that the fire of percussion shrapnel will be effective against troops defending any ordinary buildings.

Good effect has also been obtained with it against guns and personnel behind shields when direct hits are obtained.

11. The action of percussion shrapnel differs from that of time shrapnel, for the shell opens after graze, having an ascending angle, and a velocity considerably lessened by the retardation on graze. Its effect depends largely on the nature of the ground at the point of impact; in soft or marshy soil the shell are smothered and results are usually poor.

12. Percussion shrapnel, even at short ranges, must be burst very close to the foot of the target to be effective, otherwise the cone of dispersion passes over it, and descends in a shower some 250 yards beyond graze: consequently, a small error in range is a matter of great consequence.

13. _Time shrapnel._--Time shrapnel is used against living targets, against aircraft and balloons, and for ranging.

14. The best effect from time shrapnel fire is obtained when the trajectory passes through or close to the target, and the position of the mean point of burst should be as close as possible to the target without entailing an undue proportion of grazes. To attain this “ranging” must be carried out till the correct range and length of fuze are determined. (_See_ Sec. =207= _et seq._)

=111.= _Star shell._

Star shell are carried by mountain artillery. They contain a number of stars and are fired with T. fuzes. When the shell bursts these stars are ignited and illuminate the foreground.

=112.= _Time and percussion fuzes._

(_See_ Handbooks for description of various fuzes.)

1. Fuzes manufactured under varying atmospheric conditions have variable rates of burning; care should, therefore, be taken in packing limbers and wagons that fuzes of the same thousand should be as far as possible together, so as to obtain uniformity of results.

2. The rate of burning of a time and percussion fuze is influenced by the climate in which it has been kept, and the pressure of the atmosphere.

3. Fuzes also burn longer as the height above the sea level increases, that is, as the height of the barometer decreases. For each fall of 1 inch in the barometer (corresponding to about 1,000 feet in height) the time of burning increases by ¹/₃₀.[8]

[8] NOTE.--¹/₄₄ for the No. 80 fuze.

4. The _mean error_ in the time of burning of the No. 80 time and percussion fuze in use with the 18-pr. Q.F. gun may be taken as about ·14 seconds.

This error represents a distance of 48 yards at 2,000 yards range, and this distance multiplied by 1·69 (= 81) gives the length of the zone which will contain 50 per cent. of the bursts. If 10 per cent. of the bursts are on graze, the distance of the mean point of burst from the target should be equal to half the length of the 80 per cent. zone, since 10 per cent. must be taken off the other end in order to keep the mean point of burst in the same place. The 80 per cent. zone is equal to the 50 per cent. zone × 1·90, _i.e._, 81 × 1·90 = 154. Thus at 2,000 yards, the mean point of burst should be approximately 80 yards short, and at 5,000 yards 55 yards short. Though with the 13-pr. Q.F. the error of the fuze is rather greater than with the 18-pr. Q.F., the same data may be accepted. A larger percentage of grazes must, however, be expected.

=113.= _Fuze indicator._

1. The object of the fuze indicator is to give the correct fuze setting for effective burst at any range, when once the instrument has been adjusted for _one_ range.

2. _Theory._--When the graduation 150 on the corrector scale is opposite the arrow on the fuze scale slider, the length of fuze opposite each range is that which will give “_an effective point of burst_” under normal conditions. It will be noted that any corrector setting found suitable with any particular range will be approximately correct for all ranges under like conditions.

To arrive at this result the indicator has to be graduated on the same principle as a “slide rule.” The fuze scale is graduated in such a way that the linear spaces occupied are proportional to the logarithms of the times of flight. The yard scale is similarly graduated. This explains how in the table in para. 5 below an alteration of corrector makes a proportional correction at various ranges and not an equal correction throughout.

3. In the case of the 18-pr. Q.F. the effective points of burst are taken to be as follows:--

At 2,000 yards 80 yards short.
At 3,000 ” 70 ” ”
At 4,000 ” 60 ” ”
At 5,000 ” 55 ” ”
At 6,000 ” 50 ” ”

These distances represent approximately an angular height of 10 minutes at all ranges.

The corrector settings likely to give effective points of burst at various altitudes are with the bar indicator as follows:--

At sea level, or with barometer 30 Corrector 150.
At 1,000 ft. altitude or with barometer 29 ” 144.
At 2,000 ” ” ” 28 ” 138.
At 3,000 ” ” ” 27 ” 132.
At 4,000 ” ” ” 26 ” 126.

4. Alterations in the barometer will affect the corrector setting, the normal being when the barometer stands at 30. A fall in the barometer necessitates a decrease, and a rise in the barometer an increase in the corrector setting.

5. Different corrector settings give different heights of burst. To raise the point of burst the corrector settings have to be shortened, and to lower the point of burst the corrector settings have to be lengthened. The amount of the alteration in the corrector settings depends upon the range and the amount that it is desired to alter the point of burst.

By altering the corrector setting from 150 to 140, the fuzes may be expected to burst as follows:--

At 2,000 yards range 70 yards shorter.
At 3,000 ” ” 100 ”
At 4,000 ” ” 125 ”
At 5,000 ” ” 150 ”
At 6,000 ” ” 175 ”

It will be seen, therefore, that even smaller alterations than 5 in the corrector settings may be made at long and distant ranges, when it is desired to alter the point of burst of shell already bursting in the air. (_See_ Sec. =227= Examples.)

6. Comparing the first and third of the above tables it will be seen that 150 corrector at 2,000 yards gives bursts approximately 80 yards short and that an increase or decrease of 10 in the corrector varies the burst 70 yards. Consequently if it is desired to bring the burst on to the line of sight (as in time shrapnel ranging), the corrector must be increased by an amount equal to

80
---- × 10 (_i.e._, 10 for each 70 yards) = 11.
70

7. The following tables show the approximate difference in corrector settings necessary to effect an alteration in the angular height of burst at various ranges:--

------+--------------+----------------+-------------+---------------
Range.| 13 and 18-pr.| 4·5-inch Q.F. | 60-pr. B.L. |15-pr. Q.F.
| Q.F. |Howitzer ranging| | and
| | in yards. | |15-pr. B.L.C.
| | All charges. | |
| | “Gun range.” | |
------+--------------+----------------+-------------+---------------
| To raise |To raise height | To raise | To raise
| height | 20 minutes. | height | height
| 10 minutes. | | 15 minutes. | 12½ minutes.
------+--------------+----------------+-------------+---------------
1,000 | | 20 corrector. | |
2,000 | 11 corrector.| 10 ” | | 13 corrector.
3,000 | 7 ” | 6 ” |24 corrector.| 8 ”
4,000 | 5 ” | 4 ” |18 ” | 5 ”
5,000 | 4 ” | 2 ” |12 ” | 4 ”
6,000 | 3 ” | | 9 ” | 3 ”
7,000 | | | 6 ” |
8,000 | | | 5 ” |
9,000 | | | 4 ” |
------+--------------+----------------+-------------+---------------

4·5-inch Q.F. Howitzer.
_Ranging in degrees._

------------------------+---------------------------
Elevation (all charges).|To raise height 20 minutes.
------------------------|---------------------------
5 degrees. | 16 corrector.
10 ” | 10 ”
15 ” | 6 ”
20 ” | 4 ”
25 ” | 3 ”
35 ” | 2 ”
------------------------+---------------------------

Officers should know the figures for the equipment with which their unit is armed.

8. _Use._--When a corrector is ordered, the fuze scale slider is moved till the arrow is opposite the required graduation, and clamped. The fuze for any range can now be read off. For the purpose of recording the particular range and so avoiding possible error, the sliding pointer on the top can be moved and set to such range.

Once having found the corrector setting, it is the “corrector setting for the day,” provided that, if indirect laying is employed, the angle of sight is correct. If, when finding the corrector setting, the angle of sight is incorrect, the corrector setting found will be a false one, being too short if the angle of sight is underestimated, and _vice versâ_.

9. To obtain the corrector setting, an échelon of three rounds may be fired, each round at a different corrector setting. When selecting the échelon of corrector settings an endeavour should be made to choose such lengths of corrector as will give bursts in air and on graze.

10. Bursts should always be judged with reference to the “_line of sight_,” otherwise when the target is situated on sloping ground, an unsuitable length of corrector may be selected, as rounds bursting in air short of the target below the “_line of sight_” would, with the same corrector, give bursts on graze when the trajectory passes through the target.

=114.= _Range tables._

1. Range tables represent the ordinary performance of the gun with service ammunition under normal conditions. They can, therefore, only be taken as a guide. With the object of compiling these tables and of finding out, in a general way, the relative accuracy of the service ordnance and ammunition, series of rounds are fired at varying elevations for range and accuracy.

From these series mean ranges and deviations are obtained for each elevation; the difference of each round from the “_mean_” gives the “_error_” and the mean of the errors of the series gives an estimate of the accuracy.

2. The chief causes of inaccuracy, which may exist on the experimental practice ground, where all the conditions are most favourable, are as follows:--

i. Want of accuracy in the gun, faulty ammunition, or
unsuitable mounting.
ii. External causes, such as wind, and varying density
of the air.

Errors due to the gun will arise if the twist of rifling is unsuited to the length of the projectile.

Errors due to the projectiles will arise if their density has not been distributed in the best manner, or if they are inaccurately centred or vary in weight.

Errors due to the charge can be reduced to a minimum by using explosive of the same lot throughout the experiment, and by giving each charge the same air space to secure uniformity of loading density.

Errors will also arise if the mounting, or the gun on its mounting, is unsteady when the gun is fired, as this would cause a variable “_jump_” and consequently a variable angle of departure.

3. The following is an example of one series:--

------+---------+-----------+---------+-----------
No. | Range. |Difference |Deviation|Difference
of | | from | right. | from
round.| | mean. | | mean.
------+---------+-----------+---------+-----------
| yds. | yds. | yds. | yds.
1 | 4,968 | 22·8 | 24·4 | 3·0
2 | 4,954 | 8·8 | 21·6 | 0·2
3 | 4,962 | 16·8 | 22·8 | 1·4
4 | 4,908 | 37·2 | 20·0 | 1·4
5 | 4,934 | 11·2 | 18·4 | 3·0
------+---------+-----------+---------+-----------
| 24,726 | 96·8 | 107·2 | 9·0
+---------+-----------+---------+-----------
Mean | 4,945·2 | 19·4 | 21·4 | 1·8
------+---------+-----------+---------+-----------

The second column in the above table gives the actual ranges. The mean range is obtained by adding all together and dividing by 5, since 5 rounds were fired.

The third column contains the difference of each round, _irrespective of sign_, from the mean range just found. The mean of these differences is then obtained, and called the mean error in range or mean longitudinal error. Evidently, if all the projectiles fall nearly at the same range, this mean error must be small.

The fourth column gives the lateral deviation from the direction in which the axis of the gun points; the mean deviation is at the bottom of this column. If any shot had fallen to the left, then the deviation would be reckoned from a line passing through either the extreme right or the extreme left shot.

The fifth column gives the difference from the mean deviation, with a mean at the bottom called the mean error in deviation or mean lateral error.

Collecting the results from the table we have:--

Mean range 4945·2 yards.
Mean longitudinal error 19·4 ”
Mean deviation right 21·4 ”
Mean lateral error 1·8 ”

It can be shown by the theory of probabilities that if each mean error is multiplied by the factor 1·69, the breadths of zones (of infinite length), which will contain 50 per cent. of the hits, are obtained.

4. As a result of these and other tests the following information is embodied in the various columns of the range table.

i. _Elevation and Range._--The angle of elevation in degrees and minutes is shown for every hundred yards up to the limit of effective range of the gun.

ii. _Fuze Scale._--The fuze scale column shows the graduation at which the fuze must be set due to the time of flight for the range shown on the table. In the range table for the 18-pr. Q.F. we find for a range of 4,000 yards the fuze graduation is 12·8. As the fuze scales are compiled for a barometric pressure of 30 inches, this means that a shell with a fuze set at 12·8 will burst at 4,000 yards from the gun when fired at the sea level under normal conditions. For various reasons mentioned in Sec. 112, the fuze scale can only be taken as a guide.

iii. _Angle of Descent._--The angle of descent is shown either in degrees and minutes or as a slope.

As a field gun has only one charge, the only way to increase the angle of descent is to increase the range. With field howitzers varying charges are used, so that any required angle of descent can be obtained by a judicious selection of the position of the gun and the charge to be used.

iv. _Remaining Velocity._--This column shows the actual velocity of the projectile at any given range.

v. _Five Minutes Alteration of Elevation or Deflection._--These columns show what alteration in the range or point of impact is caused by an alteration of 5 minutes in elevation or deflection. The former is useful in ranging a howitzer battery, and the latter is useful in calculating the deflection to be given to concentrate the fire of the guns of a battery on to one point, and also in calculating the correction required to get parallel lines of fire when using an aiming point (_See_ Sec. =122=.) Thus, suppose the range to an aiming point is estimated at 2,800 yards, and the virtual displacement of the gun 16 yards, the displacement difference will be 20 minutes, as the range table shows that 5 minutes deflection alters the point of impact 4 yards at that range.

vi. _Time of Flight._--Time of flight is shown in seconds.

vii. _Deflection for Drift._--The sights of some howitzers not being arranged to counteract the drift of the projectile, it is necessary to give deflection. The amount required at the various angles of elevation is shown in the table. It is always given to the left.

viii. _Accuracy Tables._--These columns give the length or breadth or height within which 50 per cent. of rounds should fall and are based upon actual practice.

_Example._--As an example take the 18-pr. Q.F. gun. At 3,000 yards the 50 per cent. length is given as 20 yards, and the breadth as 1·44 yards. A series of rounds might fall, as in Fig. 16, round about a target T. Of these 50 per cent. would be contained between the two lines AB, CD, 20 yards apart, 10 yards on each side of the centre of the group. All the rounds should fall within two lines four times that distance apart, that is 40 yards on each side of the centre of the group.

Again, of the rounds that fell right and left of the centre of the group 50 per cent. would be enclosed by the two lines FE, HG, Fig. 17, 1·44 yards apart, and all should be contained by two lines four times that distance apart.

If now one pair of parallel lines are placed over the other as in Fig. 18, evidently the rectangle enclosed by them will contain 50 per cent. of 50 per cent., that is 25 per cent. of the total.

The width of other zones (containing a different percentage of hits) can be obtained by multiplying the width of the 50 per cent. zone by a varying factor (_see_ Text Book of Gunnery).

If the target is a vertical one, the figures in the column “_height_” should be taken, instead of those under “_length_.”

5. It is important to realize the effect of these laws in cases of ordinary practice, where errors in range are of chief moment. We have seen that at 3,000 yards, with the 18-pr. Q.F. gun, a series of rounds would fall within two lines 40 yards on each side of the centre of the series; therefore a shell falling 39 yards short is within normal limits of error, but it should be counterbalanced by other shots falling over.

6. Accuracy of fire is a comparative term; it is said to be good when a group of projectiles, fired under as nearly as possible the same conditions, falls close together.

The probable percentage of hits obtainable from an 18-pr. Q.F. on a shield 4 feet 6 inches high and 5 feet wide, fired under experimental conditions,

is at 2,000 yards 60 per cent.
2,500 ” 33 ”
3,000 ” 16 ”
3,500 ” 8½ ”
4,000 ” 5 ”

Under service conditions these percentages would be considerably reduced.