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calibre is increased by 5-8, and the widtlvof the grooves as compared with that of the lands is increased, the lands being reduced to the lowest limit necessary to withstand shearing. The advantages of this system are that less copper is cut away by the lands, thus leav- ing more to withstand the wear in rotating the projectile; and that the total bearing surface of copper against the lands is larger, due to the larger number of grooves.

Theoretically the same twist should be suitable for high and low velocities with a given projectile; but the determination of the ap- propriate twist must, to a large extent, be settled by experiment. In the British service, a twist of one in 30 cals. is adopted for guns, and a sharper twist varying from one in 15 to one in 25 cals. for howitzers. In the French service it is considered that with an angle of rifling 1 larger than about 7, the projectile would be given too much spin- stability, and would find difficulty in adapting itself to the trajectory when it commences to fall.

In American field guns the rifling has approximately 8 grooves per in. of calibre and is about 0-02 in. deep. In Germany, on the other hand, the changes in rifling appear to have been in the direction of narrower grooves and wider lands while the number of these has been increased ; the twist of rifling is uniform.

Obturation. In the British service, ordnance are divided broadly

into two classes: (l) breech-loading (B.L.) and (2) quick-nrin: (Q.F.). Originally the division was in accordance with the rate o fire; but developments in breech-mechanisms and mountings have

been such that, generally speaking, a modern B.L. gun is capable of a rate of fire as high as that of a Q.F. gun of equal calibre; so that, though still used, the terms B.L. and Q.F. are applied to guns ac- cording to the obturation, i.e. means adopted for preventing escape of gas at the breech. With B.L. guns the obturation is effected by the de Bange system (see 20.206). With Q.F. guns the sealing is performed by a metallic cartridge-case.

The principal advantages of the Q.F. system are: (i) the charge being in a metal case " fixed " ammunition can be used, thus in- creasing the speed of loading; (2) the cartridge contains its own means of ignition, again tending to increase the speed of loading; (3) if one case fails to obturate correctly, subsequent rounds are not necessarily affected. On the other hand, the disadvantages are: (l) it is not suitable for guns of large calibre owing to the excessive weight and length of the cartridge-case required; (2) the chamber must be of a definite shape and size and must be very accurately machined ; (3) some form of cartridge-case extractor is required.

The advantages of the de Bange system (pad obturation) is its simplicity, no preparation of the gun being required beyond the machining of the obturator-seating. Its disadvantages, apart from the slower rate of loading, are that (i) pads are liable to lose their shape unless constantly attended to, and are susceptible to changes in temperature, becoming soft in hot weather, and hard and inelastic in cold weather; (2) prolonged firing causes overheating, softening and inefficiency of the pad; (3) if a pad fails to seal completely on any one round, the result is damage to the pad and scoring of the obturator-seating, which will increase with every subsequent round unless immediate steps be taken to remedy the defect.

Breech- Mechanisms. The essential features of the breech-closing arrangements are safety, efficiency and rapidity; and the breech- mechanism must be so arranged as to embody these features as well as, at the same time, convenience in handling, which must also be studied, to admit of the mechanism being operated with a minimum amount of effort. A point of importance, as regards safety when a breech-screw is used, is to ensure that there be sufficient effective bearing surface of the screw in bearing with the breech-bush to enable the screw-threads to withstand the pressure on firing. This point affects the length of the screw and consequently the length of the breech, the heaviest part of the gun. Rapidity of action and ease of operation are chiefly mechanical questions and do not affect the design of the gun as such to any appreciable extent.

As regards carriages for light and medium field equipments, the factors which require consideration as affecting mobility or lightness of draught are as follows:

Weight. The draught necessary varies directly as the weight and inversely as the wheel-diameter. Thus the weight must be a minimum consistent with strength.

Wheels. The wheel-diameter should be as large as possible within practical limits, and the breadth of tire sufficient to prevent the wheel from sinking deeply into soft ground. When possible, similar wheels should be used on gun-carriage and limber to ensure interchangeability ; but this is impracticable as a rule in the heavier- wheeled carriages, in which the limber-wheels support only a small proportion of the weight. The wheels must be designed for the load they have to support. In the British service the latest light field wheel is 4 ft. 3 in. in diameter; earlier equipments used wheels 4 ft. 8 in. or 5 ft. in diameter; the heavy field wheel is 5 ft. in diameter.

1 The angle of rifling is the angle which the twisting groove makes with the direction of axis of the gun. It is given by - =tan. 9 when

x is the number of calibres in which a complete turn is made. Thus a uniform twist of one turn in 30 cals. means an angle about 6.

Other wheels of both wood and steel, and of larger diameter, are used with heavier tractor-drawn equipment. The mean diameter of the axletree-arm should be a minimum consistent with strength in order to reduce friction.

Wheel-Tracks. Generally, the wider the track, the larger the upsetting angle and the greater the lateral stability, but a wide track tends to increase the draught, permits of side-play of the pole, and adversely affects wheeling or reversing operations. Thus the track should be narrow, consistent with lateral stability. The track of the limber and carriage-wheels should be the same, so that the limber-wheels may make the tracks for the carriage-wheels which support the greater weight ; this tends to lighten the draught.

Limber and Coupling. The trail in travelling acts simply as a connector. It should be short for compactness, thus enabling the carriage to be reversed within a small space; and narrow towards the trail-eye, allowing a large angle of lock and thus ease of ma- noeuvre. The coupling must, for travelling over rough country, per- mit of limited vertical and twisting movements of the limber with respect to the carriage, or vice versa; and it must preserve the balance of the pole. The coupling in general use is the trail-eye and limber- hook which allow of free play of one with respect to the other; sometimes the eye is swivelled to permit of a rocking movement which further increases the flexibility of connexion.

For ease of draught there should be a slight reduction in weight on the limber-wheels as compared with that on the carriage-wheels, the ideal proportions varying between 2 to 3 and 5 to 6. With light equipments the total load is not rendered excessive by carrying am- munition in the limber, and this gives a suitable distribution for travelling. With medium horse-drawn equipments, it is undesirable to add weight with ammunition, but by hauling back the breech of the gun to a travelling position upon the trail a suitable distribution is obtained without increasing the load. With tractor-drawn carriages particular distribution is not necessary.

Balance of Pole. It is important to take the weight of the pole off the horses as much as possible, and the limber is designed with this object in view. With medium equipments, the balance must be preserved while the gun is transferred to the travelling position; this necessitates a special coupling, known as a limber-connector, which is flexible and also supports the weight centrally on the limber- axle. The load is pulled from and supported by the limber-axle, so that the variation produced when distributing the load will not disturb the balance of the pole.

Other necessary elements of a carriage design are brakes which serve the usual purposes in travelling, and travelling clamps which lock the portions of the superstructure to the travelling support in order to reduce play.

To permit of the necessary three movements of the gun in action, a superstructure must be built up on the basic structure of the carriage. For traverse a component, known as an inter- mediate carriage, is pivoted vertically to the basic structure, so that lateral movement can be obtained through traversing gear. With light and medium carriages the intermediate carriage is pivoted near the front of the trail, with others to the platform. The most notable exception is the case where cross-axle traverse is embodied in the design, when the inter- mediate carriage is dispensed with. The front of the trail is moved bodily along the axle by suitable gearing; the sides of the trail are extended upwards to support the cradle. This method has an advantage in that the direction of recoil is always coincident with the centre-line of the trail; but the necessary wheel and axle movements tend to render traversing difficult.

For elevation, the intermediate carriage must be designed to carry, pivoted on a horizontal axis, a cradle which, under the control of elevating gear, can give the required elevation to the gun; the cradle is carried by trunnions in plain or roller bearings at the top of the intermediate carriage.

For axial movement, the cradle must provide a support on which the gun can slide, but must prevent it from having twisting movement; it must also carry the recoil system. Thus the cradle and gun must have guides, and the gun also a lug for attachment to the recoil system.

The superstructure thus consists in an intermediate carriage, a cradle, a recoil system, and gears for traversing and elevating.

During action, complete immobility of the firing support is required; this means that the carriage 'must be secured against backward, forward, side and turning movements. In a typical light carriage, backward movement is prevented by the spade; forward movement by using the brakes in conjunction with the spade; the side movement by limiting the traverse. For turning movement the standard aimed at is that the wheels shall not lift during recoil, nor the trail during recuperation.