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Conversion. An existing gun design does not as a rule lend itself very successfully to conversion to another pattern, though the expedient is sometimes, from force of circumstances, resorted to. It is seldom that the power of a gun that is, its range and accuracy can be improved by alteration; but by altering the breech-mechanism or adapting the gun to suit a new carriage it may be possible to augment its tactical, if not its ballistic, power. As examples of conversions which have been carried out in recent years in various countries may be cited: the conversion of field guns from B.L. pattern to Q.F. pattern in order to avoid the expense of complete rearmament; the con- version of old-model guns and howitzers by cutting down and boring out, during the World War; and the very frequent expe- dient of remounting the piece on a new type of carriage.

As the requirements demanded of a carriage in the field become more exacting, so the design becomes less simple, and any adaptation of an earlier design more difficult. At certain limits it becomes more economical to build a new design.

In recent years, changes in design have been more frequent and far-reaching than during many years previously. Conse- quently, formerly the question of obsoleteness of an equipment was one which did not have a very definite meaning. The task imposed on a gun in the past was neither so severe nor so definite as at the present time; and so the idea of what constitutes obsoleteness has only lately required to be expressed explicitly. It may be taken nowadays that an equipment is designed for a certain definite purpose, and that when it can no longer perform the duty required of it, it has become obsolete. Nevertheless, to apply this proposition as a test in any given case is much more difficult than to enunciate it. Thus, during the World War, so many new designs of carriages were built on quite new lines and embodying quite new ideas, that it might be said that all carriages of earlier date are now obsolete. But such a con- clusion does not necessarily apply to all natures of carriages; for instance, though with light field carriages any spring-recuperator is obsolete, it does not follow that with a fixed mounting spring- recuperation is not efficient. It is difficult to draw any decided line or define obsoleteness to cover all classes of carriage and mounting. All that can be said is that a system or mechanism, formerly employed, which no longer satisfactorily and fully fulfils the ruling conditions of any class of carriage, may be considered as obsolete for that particular class.

Nevertheless, though a method or design may become obsolete, the question of stocks has an important bearing on whether the equipments of obsolete design may be discarded; and this final decision rests almost entirely on the capability of the equipment to give reasonable satisfaction in the ballistic requirements.

From all the above it will be seen how intimately the elements of a design react upon one another, and how in its final version the design becomes a compromise between conflicting require- ments and limitations. The table below collects some of these requirements in the case of field carriages. Juxtaposed, they indicate the nature of the designer's problem without the need of further comment.

(II.) MANUFACTURE

General Method of Building a Gun. In the general methods of building a British gun very little change has taken place in recent years. The gun is either built wholly of steel tubes shrunk together, known as the " all-steel " construction; or the inner tube is reinforced by winding on layers of steel wire and

the whole encased in an outer jacket, known as the " wire- gun " construction. This latter construction is that now gen- erally favoured and the number of layers of wire and the distance from breech towards muzzle covered with wire depend upon the particular class of gun.

The 6o-pdr. gun may be taken as an example in the process of gun design. In the Mark I. gun there is an " A "-tube, lined with an inner " A "-tube extending from the seat of the obturator to the muzzle and secured longitudinally by means of corresponding shoul- ders and a breech-bush screwed into the " A "-tube at the rear; the breech-bush is prepared for the reception of the breech -sere w ; the layers of wire cover a portion about one-half of the " A "-tube, extending from the breech end; the jacket covers the wire and ex- tends towards the muzzle along about one-half the unwired portion of the " A "-tube, and is secured longitudinally by corresponding shoulders; the breech-ring is fitted over the rear end of the " A "- tube and is screwed to the jacket ; the breech-ring is prepared for the reception of the breech-mechanism and is provided on the upper side with lugs for the attachment of the hydraulic buffer and running-out springs of the carriage; longitudinal ribs on each side of the jacket form guides for the gun in the cradle.

In the next mark of 6o-pdr. the " A "-tube was made thicker and the inner " A "-tube omitted; and this was also the case in the Mark II. which was introduced as a more powerful gun, being some 5 cals. longer than the Mark I. The last mark, however, has an inner " A "-tube extending from the seat of the obturator and slightly projecting from the muzzle; the interior of the "A "-tube is pre- pared with longitudinal grooves to prevent any turning movement of the inner " A "-tube when in position; a lug is provided on the under-side of the breech-ring for the attachment of the hydro- pneumatic buffer of the carriage; and projections on the under-side of the jacket are formed to fit the slides on the cradle.

The measurements of the chamber of the 6o-pdr. are: largest diameter, 6 in.; smallest diameter, 5-3 in.; length 21-926 in.; and the total length of the latest pattern gun is 192-25 in.

Recently another method of building a gun, termed " auto- frettage," originating with the French, has been tried as an alternative to shrinkage, but it is still in an experimental stage.

The rule hitherto has been to limit the greatest strain developed in a gun-body to the value corresponding to the strain produced by a tensile stress equal to the elastic limit of the material. Beyond that limit permanent deformation takes place. But it is now argued that with any static load less than that which produced an existing permanent deformation, the total deforma- tion will remain less than the maximum reached under the latter, and the permanent strain does not increase; so that a member statically stressed to a point quite beyond the natural elastic limit would be entirely safe under any static load extend- ing below that point, and that the point may be considered as a new elastic limit and adopted as such for practical purposes. As applied to a hollow cylinder or a gun, auto-frettage consists in applying an interior pressure, preferably by hydraulic power, of such intensity as to cause deformation of the inner layers much beyond that corresponding to the natural elastic limit. The permissible interior deformation is based upon the ratio of outer and inner diameters, upon the physical qualities of the steel, and particularly upon the paramount condition of keeping the inner layers free from fissures and breaks. Precautions must also be taken so that, upon removal of pressure, the material of the inner fibres, which will then be under intense compression, will not be further deformed and weakened by these compressive stresses. In action i.e. under interior pressure all the layers are in a state of tension varying, of course, with the radial position of any given layer. On returning to rest, all the layers tend to return to original lengths, but because of the permanent deformation, intentionally produced,

Element.

Weight

Trail length. .

Trail width

Wheels diameter. Centre of gravity 1 Centre of height / ' Axle length.

Travelling requirements.

Small for mobility.

Short for compactness when

limbered up. Narrow at end for angle of lock.

Large for mobility.

Affected by diameter of wheels.

Long for lateral stability, short for other points.

Firing requirements.

Large for stability. Large for stabiltiy.

Wide on axle to reduce inten- sity of shock.

Small for stability. /Low for stability, high for\ \ command of fire. /

Sufficient to give required tra-

Governing factors.

Travelling. Firing.

Firing.

Travelling.

Stability and command of fire.

Travelling.