All gun body members are made of steel (q.v.) of a specified chemical composition and physical properties. The more important tubes are usually of a high grade alloy steel, the essential physical property being a high yield point to give a good elastic range. The British specification requires the steer to be made by the acid open-hearth process to ensure the pure quality of the ingredients. The steel is cast as an ingot which in the case of large forgings may weigh i oo tons or more. There is a large discard from the top and a small discard from the bottom of the ingot and the core is removed by trepanning before or after shap ing up the ingot into a billet.
The material is then forged under a hydraulic press on a water-cooled mandrel in a series of heats to draw it into a tube of the required dimensions. In this forging operation the length may be increased five times and the diameter halved. The forging, after straightening and rough machining, is nor malized or annealed in a furnace to remove the forging stresses. Oil hardening is then carried out by heating the forging to a suit able degree and immersing it in oil, after which it is tempered as necessary and the required properties are tested mechanically by breaking and bending test pieces taken from the two ends of the tube.
The tubes are machined to the finished dimensions for building, the exterior of an inner tube being turned to conform to the measurements given by the bored interior of the corresponding outer tube, allowing for the design shrinkages. The outer tube is heated sufficiently to obtain the necessary clearance by expansion and is then lowered over the inner tube or gun which is placed vertically in a pit for this pur pose and kept cool by water running through the bore. As soon as the outer tube is properly in place, that is, the rear or building shoulder is in contact, it is rapidly cooled from this shoulder which is near the breech end by the application of water. This cooling requires careful control to prevent the tube seizing at any f or ward position. The forward portion is kept heated by gas rings, which are gradually withdrawn as the cooling proceeds, and the water moves up the exterior of the tube, thus ensuring that the tube is held or drawn towards the breech end throughout the shrinking operation. In the case of a wire wound gun, after inser tion of the liner or inner "A" tube, which is slightly conical and a driven fit, the rear end of the "A" tube is threaded inside to take the breech bush (see fig. 9). A stepped collar is shrunk over the rear end of the "A" tube. This reinforces the breech bush and forms the connecting link between the "A" tube and jacket through the medium of the breech ring so that the longitudinal stresses can be suitably distributed. The muzzle stop ring is
shrunk over the muzzle of the "A" tube. This ring forms the front support for the wire which is wound over the "A" tube. The wire consists of a steel ribbon a in. wide and .o6 in. thick. The breaking strength of this material is over roo tons per square inch and all the wire used is, previous to winding, subjected to a very severe load to ensure that it is perfectly sound. The fact that wire can be so tested throughout is a great merit of this form of construction. It is wound on the gun in a continuous length, the number of turns varying in the case of a 15 in. gun from 20 at the muzzle end to 79 at the breech. Approximately 2 2 tons of wire, about 186 miles in length, are required for such a gun. The winding tension is such as to produce the required design tension after completing the winding operation, allowance being made for the effects of subsequent layers of wire on those already wound. This tension is controlled during winding by a readily adjustable friction brake. On completion of the wiring, the external layer of wire is turned with a fine cut to obtain a good building surface for the '13" tube and jacket which are shrunk on. The breech ring is finally screwed and shrunk on. The exterior of the gun is finish machined and the bore and cham ber are bored and lapped to size.
gun is then rifled : the spiral grooves are cut by a special machine, several grooves being cut at the same time, to the required depth and form. The number of grooves, their depth and general form and the relative width of the groove and land (intervening bore sur face) vary considerably in differ ent guns. The general form of groove is one of plain section in which the width of "land" is ap proximately half the width of the groove (see fig..i 2 which contrasts
and earlier types of rif ling). The twist of rifling is generally uniform, one turn occurring in a length of 3o calibres, but increasing twist is also used and a combination of increasing and uniform twist. A typical modern form of driving band is also illustrated. It has three functions to perform :—(i) To rotate the projectile. (2) To act as a gas seal and so prevent the bore of the gun being scored or guttered by gas escaping over the projectile, which would result in irregularity in the muzzle velocity. (3) To hold the projectile at any elevation after loading so that it does not slip back into the chamber, and to ensure that a certain pressure is generated before the projectile starts to travel.