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guns. The possibility of repair, when worn out, has considerable bearing on design, for where this is required the inner tube, or barrel, is made in two parts, one inside the other, so that when necessary the inner one can be removed and replaced. The British practice is that all guns, except very light guns, are built with an inner " A "-tube. The probable life of a gun is estimated by the number of rounds it can fire before being worn out and depends on many factors, the principal of which is the action of the powder gas. In the actual design it is affected by the material to be used for the inner tube and by the shape and size of the chamber.

Problem of Design. The problem before the gun-maker is to build a gun under denned limitations of weight, etc., that will withstand certain definite pressures at various parts of the bore. Steel is practically the only material now used in gun construction; but there are several classes or kinds of steel, and experience has taught which particular class can be obtained and used for the various parts of the gun. In addition to con- siderations connected with the design of the gun proper, there are two important points which require to be considered and embodied, namely, ease and rapidity of loading and firing. These two points necessarily involve the determination of the kind of breech-mechanism and the class of obturation to be employed. The weight of the "moving parts of the breech- mechanism has a serious bearing on the ease of manipulation; and the mechanical device chosen for opening and closing the breech and the complication of the mechanism for holding and firing the means of ignition regulate very largely the rapidity of the service of the gun.

Carriages. An artillery carriage must be designed primarily to provide a support from which the gun is fired; but in some cases the carriage is required to perform also the function of transporting the gun to the firing position. The demand is, therefore, for either a fixed or mobile carriage; and in the latter case it is accepted as a necessity in modern mountings that the under-portion, or basic structure, should remain immobile on firing. Since guns vary greatly in calibre, power and weight, the mobility of the carriage and the method of draught adopted must vary accordingly, because the weight and size will be determined largely by the gun for which it is designed; but, consistent with the functions required of it, the weight of a carriage should be a minimum and its mobility a maximum. The ideal of providing the travelling as well as the firing support in the carriage of any mobile gun is possible of attainment only with light and medium guns, owing to the limitation in weight for travelling; and the various designs of mobile carriages are a direct outcome of the necessity for mobility of loads varying from about 2 tons upwards.

Movements Necessary. From the firing aspect, the modern requirements of any carriage necessitate that the gun shall be laid (i.e. aimed) quickly and accurately without any movement of the basic structure, and that the basic structure shall remain completely immobile during firing. Thus any carriage must be designed to permit of three distinct movements of the gun:

(a) A. movement in the horizontal plane termed " traverse " to give direction to the gun. This movement is limited with all mobile carriages, due to the fact that the side stability of the carriage must be maintained. Generally, the heavier the gun and carriage, the more extended must the traverse be, in order to avoid the neces- sity for frequent realignment of the basic structure for changes of target. With fixed mountings it is obviously necessary to have very wide and, indeed in most cases, all-round traverse, and this is ob- tained, so far as the constructive limitations of the mounting permit, by mounting the superstructure upon a central pivot under its centre of gravity.

(6) A movement in the vertical plane termed " elevation." The elevation permitted to a gun determines its range and is, there- fore, a determining factor in the design of the carriage. In recent years, increased range has been obtained with existing ammunition by removing from new designs those constructive limitations which limited the elevation in earlier types. With guns, the maximum range is obtained at about 43 of elevation; and with hyper-velocity guns (muzzle velocity about 5,000 f.s.) at about 55 of elevation; with howitzers, elevations up to 70 may be required, to give steep angles of descent ; while an anti-aircraft mounting should be capable of giving elevations up to 85 degrees.

(c) An axial movement of the gun termed " recoil " the re- verse of which, the returning of the gun to the firing position, is termed " recuperation." The latitude it is necessary to allow here depends on the type of carriage. All mobile carriages require a relatively long recoil in order to maintain stability at the lower angles of elevation. With fixed mountings a long recoil is not so necessary, and the length is relatively short (being fixed at from 3 to 4 calibres), while the mounting itself is designed to withstand the resulting stresses, since conditions of stability do not arise.

Design Elements. In the designing of a gun system or " equipment " as it is called the controlling elements depend upon the particular purpose for which the gun is required. The gun may be for a mobile mounting or for a fixed mounting. The mobile mounting may be horse drawn or tractor drawn, or it may be carried by pack transport.

So far as the gun for field equipments is concerned, it is immaterial to the design whether the traction is by horse or tractor or whether the mounting travels as an entity or in parts. The gun is nowadays designed for a long recoil, and in itself is employed as a shield for the vulnerable recuperator which is carried in a cradle below it, 1 and above which the gun recoils along slides. For light field guns, in the interest of rapidity of fire, obturation is generally by means of a metallic cartridge case, and percussion firing is always used. To enable extraction of the cartridge case, an extractor must be embodied in, and be worked by, the breech-mechanism. As regards the breech-block for guns firing fixed ammunition, this may be either of the swinging type or a sliding wedge; but if fixed ammunition be not used, the block must be a screw and the obturation effected by an obturator-pad in the breech- mechanism, since the only practical means of obturation with a sliding wedge is by use of a metallic cartridge case.

With pack transport, the weight a mule can carry is the determining factor in the design of the gun. After deductions for the weight of the saddle equipment, etc., about 250 Ib. is all that can be allowed per mule for the gun. Since the gun must be heavier than that amount in order to obtain the required power, it must be designed to be transported in two parts which can be readily and easily put together with a gas-tight joint; and the breech-mechanism must be capable of being quickly detached and reassembled.

With fixed mountings there are not the same restrictions as with mobile mountings; but the breech-mechanism must be arranged so as to allow of the use of electric, as well as percus- sion, firing apparatus.

As regards carriages, from the travelling aspect, it has to be remembered that the weight influences the draught, and the draught influences the design of the carriage. The general travelling conditions are as follows. For horse transport, in the case of a reasonable degree of mobility and manoeuvre, six light-draught horses should not be exceeded. For heavier loads, eight heavy-draught horses may be employed, as they would be rarely required to move faster than at a walk. Pole-draught is now used universally. It follows that the limiting weights are 2j tons for light field equipments, and 6 tons for medium field horse-drawn equipments when assembled for travelling. For pack transport an equipment must be capable of being split up into loads (usually from 6 to 8) no one of which exceeds 250 Ib., and in addition fulfil the same functions as a field carriage, when assembled. Other conditions besides the weight limit have to be taken into account in designing pack artillery equipments, e.g. the centre of gravity of the load as it lies on the pack-saddle should be as low as possible, for stability. For tractor transport, if the travelling weight exceeds 6 tons, tractors must be used. A load up to 12 tons can be travelled tractor- drawn as an entity on two axles, for 6 tons is the maximum average axle-load. The total load behind a tractor should not exceed 20 tons, and steering requirements limit the number of units in the train. If it be desirable to travel super-heavy equipments by road, they must be so designed as to be capable

1 Most British guns of pre-war design, used in the World War, had the buffer and recuperator above the gun. In types designed during the war this practice has been given up.