the inner layer, for instance, would 'be set to a new length
greater than the original and smaller than that developed in
action; it is then compressed by the combined efforts of all the
surrounding layers. The result is that the whole body of the
solid hollow cylinder is in a state of compression greatest at
the interior wall and least at the exterior wall. As a gun has a
tube of thickness diminishing from chamber to muzzle in order
to conform to the variation of the powder gases, it is not possible
to employ a uniform auto-frettage pressure throughout the length
of the tube. With auto-frettage, though the elastic strength
may be raised, the point of rupture must remain the same.
The advantages claimed lie principally in economy of time,
labour and material.
Big Bertha. The method of manufacture of the German long- range gun, " Big Bertha," might be cited as an example of special adaptation. The gun was constructed from worn-out 38-cm., 45- cal. naval guns, used merely for the sake of the forgings. The con- verted gun was built in two parts, a main section about 30 metres long and a forward part about 6 metres long. The old gun was bored out and a very heavy tube of 21 cm. inside diameter inserted. Some 13 metres projected beyond the end of the original gun and over this portion another hoop was shrunk and locked to the forward hoop of the old gun. The main portion was rifled with uniform twist ; the forward section, attached by means of interrupted threads and heavy outside collars as flanges, was not rifled but of a diameter larger by the depth of the grooves. The reason for the extra smooth-bore sec- tion was probably to align the axis of the projectile more perfectly with the axis of the gun and so to reduce to a minimum any irregular- ity of the projectile as it left the muzzle. The inner tube was made of such a thickness that, when worn too far, it could be bored out, in situ, to a larger diameter and the gun again used with a larger projectile; and this operation could be carried out twice.
With the 6o-pdr. gun the rifling in the Mark I. was of polygroove plain section of uniform twist, one turn in 30 cals., with 24 grooves. In the latest mark of gun the rifling is the same shape and twist, but there are 32 grooves.
Whereas, in England, formerly the method of cutting the rifling was to cut one groove at a time, the tendency now is to cut two or three grooves at a time, as it is found that sufficient accuracy can be maintained during the working; thereby time and cost are saved. But it is thought that to cut more than three grooves at once is dangerous to the required accuracy. In America the practice of cutting all the grooves at the same time is favoured in certain instances, and with the 75-mm. field gun the method of rifling the bore enables the whole 24 grooves to be worked upon at the same time.
Recent Developments in Breech-Mechanisms. As regards breech-mechanisms, that known as the " pure couple " mechan- ism is practically the standard type for all heavy and most medium guns in the British service. Greater ease is ensured by its use, as the breech-screw is balanced by equal and opposite forces applied to the two studs on the breech-screw, thus eliminating any tendency to lateral translation of the screw, and so reducing wear and friction.
With the French " mortier de 293 mm.," automatic withdrawal of the breech-screw is effected by power stored up by the recoil of the piece. The screw is of the Welin class, and is prolonged to the rear by a tail-piece with steep threads fitting with an attachment from which an arm passes ending in a sleeve movable about an extension of the recuperator piston rod. This extension is fixed to a carrier between the arms of which lies the sleeve attachment from the breech ; a toe-piece on the movable part engages with the recuperator body and can move in a cam-groove. As the gun recoils, the recuperator body, with the sleeve interior, moves with it; the carrier sliding along the cradle passes over a pawl. On recuperation the carrier is arrested by the pawl, and the arm between the breech-block and carrier is held while the gun continues to complete the run-up; on account of the screw-thread on the breech-block tail-piece, the screw is obliged to turn and is thus withdrawn from the gun. After loading, the pawl is put out of action and the pressure of the recuperator, by forward and turning motion combined, effects the breech-closing.
With the Schneider iO5-mm. gun of 1913 pattern, the breech- mechanism has a special rapid movement. The screw is of the swinging type. Plain and thread segments alternate as usual, but the dividing lines between them, instead of being parallel to the axis of the gun, are drawn to a curve and the breech-block is hollowed on the side nearest the pivot. The carrier-ring is cranked and pivots on an axis somewhat displaced to one side. The combined effect of these arrangements is that the block can swing into and out of the breech-opening without a separate movement of translation. The
necessary motion is imparted by a lever actuating a rack on the breech-face of the breech-block which rotates teeth on the breech- block; one-quarter turn locks or unlocks the breech. The extractor arm lies in front of the breech-block pivot and is operated by it through a cam system.
The breech-mechanism for light field guns has followed generally onthelinesof those for heavier B.L. guns. The British l8-pdr. Mark I. equipment contained a novel feature, in that the breech-screw was coned throughout its whole length, the larger diameter being in front, so that when the breech was closed it had a wedge-action which pre- cluded any possibility of the block being blown out.
Another form of breech-closure is the Canet system adopted in the French 24O-mm. coast gun of 1903. In this a semicircular block prepared with screw-threads is pivoted transversely in the body of the gun. In the closed position the block is above its axis, and it is maintained there against gravity by special mechanism. On recoil, this mechanism disengages itself automatically, and the breech- block, no longer supported, swings down into the open position.
A recent type of breech-mechanism known as the Asbury mechanism has been introduced into the British service, being intended primarily for howitzers. The distinctive features of this are a vertical or fore-and-aft motion of the breech-mechanism in lever, an arrangement whereby increased mechanical power is gained at the final moment of closing the breech and at the first moment of opening it (thereby avoiding forcing and slamming) ; and a form of catch which prevents the breech-screw from revolving during the swinging motion of opening and closing. This type of mechanism has proved very satisfactory, and has been applied to howitzers of heavier natures, as well as to light and medium field guns. The advantages claimed for it are: (l) it facilitates the service of the gun by placing the breech-mechanism lever in a more convenient position; (2) it prevents slamming of the breech and eliminates damage to the threads of the breech-screw due to rebound of the mechanism; (3) it has a positive unseating action on the obturator.
With sliding wedge-mechanisms the two main types still remain the vertical or falling block and the horizontal sliding block. It is necessary with the sliding block that the guide ribs along which the block slides should be inclined so that the block, as it moves to the closed position, has also a forward movement, thus forcing the car- tridge into its seating in the chamber of the gun. The method of operating the block, in the main, remains the same as in early designs, but additional mechanism has been introduced to enable the me- chanism to be worked semi-automatically, particularly in connexion with firing at aircraft at high angles of elevation, when it is necessary to adopt some mechanical means to assist in loading, and to pre- vent the cartridge slipping back while the breech is being closed.
With the latest type of mechanism, the practice is to use separate locks for electric and percussion firing. These locks follow generally on the lines of the combined lock, but a much lighter mainspring is used in the electric lock; and the percussion lock is usually so arranged that it is automatically cocked during the opening of the breech. Guns and howitzers fitted with the Asbury type of mechanism were originally intended for firing with a vent-sealing friction tube,- no lock being required; but this method proved unsatisfactory chiefly on account of the excessive erosion of the vent. As a temporary expedient, the Martini rifle-firing mechanism was adapted to the vents of these pieces, and fired a percussion tube similar to the small-arm cartridge. It has been superseded by a lock and slide-box.
With i8-pdr. field guns the striker is designed for percussion firing only, having a trip action in which the striker is cocked and released by a continuous pull on a lanyard or a firing lever on the carriage.
Carnage Elements. The kinds of trails in use for mobile equipments may be classed as pole, box and split.
The pole trail is merely a steel tube and favours travelling rather than firing, being light, a simple connector, and permitting of a large angle of lock; its greatest defect is the limitation it imposes on the elevation of the gun.
The box trail consists of two bracket sides connected at either end; its main feature is to permit of increased elevation, as the gun can move down between the side brackets, and it is the standard type for use with modern 3-point-suppprt carriages. This type, on the other hand, does not favour travelling so much as the pole trail, as it is heavier and is necessarily wider at the travelling front, thus diminishing the angle of lock. When used with an axle-traverse equipment the trail can be narrower, since it traverses with the gun.
The split trail consists in two similar legs attached at the front by ball and socket joints to the axletree, and a crossbar; the legs are splayed outwards for firing; the flexible connexion allows of a limited vertical movement to accommodate the legs on uneven ground. This construction is necessarily heavier and hence adversely affects the mobility; its great advantage is provision of an extended traverse. The first practical example of the split trail was the Deport Q.F.
