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sufficiently forward to allow the projectile to be firmly secured in the case. The band must be rigidly secured to the projectile so that it will not become detached nor turn independently of the shell.

The material employed for driving bands should be soft enough for the band to be readily engraved by the rifling, even when using reduced propellant charges. Such material is of course easily dented, and as damaged driving bands lead to inaccurate shooting and increased erosion of the gun, care is necessary to ensure undam- aged driving bands for service. If, on the contrary, the metal be too hard, it will throw an excessive strain on both projectile and rifling. For all these reasons copper ring, cut from a drawn tube and afterwards annealed, has been found to be the best and most suitable material. A cupro-nickel band has been employed with certain high-velocity medium guns using heavy charges. Electro- lytic iron and bronze alloys have been experimented with; and during the World War, owing to the scarcity of copper, the Ger- mans tried other metals, such as zinc and white metal alloys; and with two driving bands the upper was made of copper and the lower of zinc. The Germans also tried a novel combination in which the foundation of the band was a strip of ordinary carbon steel on which was a copper covering, the two metals being so adherent as not to be separated.

Copper bands have the drawback of causing so-called "cop- pering," particularly with high-velocity guns. As the projectile passes along the bore small particles of the copper band are detached and sweated on to portions of the bore of the gun; and if this sur- plus copper is allowed to accumulate, eventually a copper " choke " results, making that particular portion of the rifling smaller than the remainder, so that if windage is insufficient to accommodate it, either the gun must expand and bulge or the walls of the projectile set in. To get rid of copper choke it was formerly necessary to put the gun out of action and by chemical or electro-chemical proc- esses dissipate the adhering copper. But recently it has been discov- ered that the copper deposit can be eliminated by using a small quantity of tin-foil between the propellant charge and the pro- jectile; the alloy melts, being reduced to extremely fine particles which are deposited in the bore of the gun; and the tin combines with the copper to form a fusible alloy which is swept away by the next discharge. What are called " decoppering rings " have been tried attached to the shell. The Germans employed strips of alloys such as tin-lead and zinc-aluminium pressed into a groove round the shell. 1 Decoppering charges or rings would be employed after the gun has been warmed by firing; and the gun must be absolutely free from grease or graphite material.

FIG. 2.

Generally the design of driving band as used by European con- tinental powers, is a narrow strip somewhat rounded on top. In the British service the band is of a more elaborate shape. The first torm of band (fig. 2, a) was narrow, flat on top and with a front slope so as easily to take the rifling, but excessive fringing of the copper caused the introduction of a broader and shallower band (b) with cannelures cut in it to receive any stripped copper. As this shal- low band was found not to grip sufficiently with worn guns, a band with a gas-check (c) was introduced, in which an undercut lip was formed on an increased diameter towards the rear part of the band ; this lip was readily expanded on firing and formed an effective seal

1 Such decoppering rings will be seen in fig. 10 below the lower driving band in (b) and (c). (C. F. A.)

for the gas. For modern heavy high-velocity guns, driving bands with a greater body of metal are found necessary so as to give a better grip to the rifling; in such designs (d) a raised hump of metal is placed near the rear of the band. Before the war the ten- dency with medium types of guns was to have a broad band with but small depth of metal between the highest point of the band and the line of the shell ; but this was modified during the war period in order to save copper, and to keep worn guns longer in use, by using a very narrow band of increased thickness.

In addition to the ordinary driving band, a band round the shoulders of a shell is sometimes used as a forward centring band to steady the projectile while passing through the bore of a gun. This band would not be of such a thickness as to be engraved by the rifling. The same result is obtained in many designs (see fig. 53) by swelling out the metal of the shell to a distinct shoulder, some- times known as a bourrelet. It requires, however, very accurate machining of the shell body at this point, and, probably for this reason, the use of a steadying band is more favoured on the con- tinent of Europe. 2

The weight of a shell is, to a certain extent, limited by its length and the stresses permissible on gun and carriage. With certain guns projectiles of different weights are used, and, provided that the difference in weight is not excessive, such shell when fired under similar conditions will have, at the muzzle of the gun, approxi- mately equal energies, though they will range differently.

With different types of shell fired from the same gun, there must be a difference in length, to ensure that the weight of all the shell is the same. To ensure accurate shooting the shortest shell should be not less than two calibres in length; while on the other hand a very long shell introduces difficulties as it necessitates a sharper twist of rifling in the gun to give the requisite rotation, and thus imposes a greater strain on the gun. A very successful German shell for a light field howitzer was about 4! calibres in length with centre of gravity about if calibres from the base; had the head struck with about 4! calibre radius; was stream-lined and weighed about 35 Ibs. 3

All modern shell are prepared to take a fuze for igniting the explo- sive in the shell (Sect. III. b.). Pointed shell (such as those intended for armour-piercing) for which the head must be retained intact, are fitted with base fuzes; other shell have the heads truncated and fitted to take nose fuzes. In addition to the fuze a " tracer " is sometimes employed in order to mark the trajectory while the shell is in flight. For night use, the tracer shows a luminous spark, for day use the tracer gives a smoky trail.

The natures of shell now used are: (a) high-explosive, (b) shrapnel, (c) armour-piercing, (d) special shell of kinds such as smoke, star (illuminating), gas and incendiary.

(a) A high-explosive shell may be looked upon as a travelling mine, containing a large disruptive charge of high explosive. To obtain satisfactory results there must be full and complete detonation of the high-explosive bursting charge within the shell, otherwise the general effect will be small.

Steel

el Plftle

Lyddite

ToVder Pellets ,

In Bag I Fuze hole

Exploders

FIG. 3.

The body (fig. 3) is normally of forged steel with a solid base; a special steel plate is fitted in a recess in the base; and a socket screwed into the head, or the head itself is threaded to take a fuze which completes the point of the shell. If the shell body be of considerable thickness the explosive content is reduced; but on the other hand the shell body is stronger and there is thick metal for man-killing splinters. A thin-walled shell with a maximum explosive content on the other hand is adapted for the attack of material; with an instantaneous fuze it is useful for clearing ground of obstacles such as wire entanglements, and,

2 A German shell with a steadying band is illustrated in fig. 10 (a).

(C. F. A.)

'A useful approximate rule for comparing the weights of shells of different guns (similarity of shape being presumed) is: half the cube of the calibre in inches is the weight in pounds. By this rule the weight of the shell just mentioned would be 34-5 Ib. and that of a similar 5-g-in. shell would be 103 Ib. (C. F. A.)