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:--