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striker rides up an incline I (fig. 45) on the box slide and thus pushes back the striker from contact with the tube. The extractor described above is actuated at the same time. Most locks consist of a steel frame with a socket for containing the striker and main spring. They are contrived so as to be capable of firing both electric and percussion tubes, but others are arranged for firing only electric, separate locks being employed for use with percussion tubes. The construction of both is very similar, but with the percussion lock, or the combined lock, a trigger is provided which drops into a notch in the striker when this is pulled back by the lugs E E (fig. 45) on the outer attachment of the striker. On the trigger being pulled by a lanyard the striker is released and fires the tube.

Fig. 40. — Breech Mechanisms, Heavy Guns.

Fig. 42.— Breech Mechanism, 6-in. B.L. Mark IV.

For Q.F. guns with interrupted or coned breech screws the striker is contained in the breech screw, but, in order to provide for safety, a small lever cam or other contrivance is fitted which, when the mechanism commences to open, is operated by the hand lever and withdraws the striker from contact with the primer inserted in the cartridge case.

The striker consists of a steel needle, with the stem insulated by ebonite or some similar material, contained in an outer steel sheath. The sheath is formed with a foot or lug which is acted upon by the safety gear; a collar is also provided for taking the thrust of the main spring.

Another form of lock now much in favour, especially for field-gun mechanisms, is that known as a trip lock. It is mainly used for percussion firing but can also be combined for use with electric tubes. In this pattern the striker is withdrawn, cocked and released by the continuous pull of a hand lever attached to the mounting or by a lanyard attached to the lock. Should a miss-fire occur the striker may be actuated as often as necessary by releasing the hand lever or lanyard and again giving a continuous pull (fig. 46).

In all modern heavy guns, especially when firing to windward, there is a tendency, when the breech is opened rapidly after firing, for a sheet of flame to issue from the open breech. It was practically unknown with the old black powders, but is of frequent occurrence with all smokeless propellants. If Back flash. the gun is loaded immediately after the breech is opened the fresh charge may be ignited and an accident caused. Several serious accidents have already been traced to this cause, notably one on the United States battleship “Missouri” on 13th April 1904, when 33 lives were lost. The flame is due to the large amount of highly heated carbonic oxide remaining in the gun from the explosion of the charge; this mixing with the oxygen of the air when the breech is opened burns rapidly as a sheet of flame in rear of the gun, and should wind be blowing down the gun the action is more intense. By looking into the gun from the muzzle, before the breech is opened, the gas can often be seen burning with a pale-blue flame as it slowly mixes with air and a curious singing noise is heard at the same time. It is now usual to fit a special apparatus on the gun, so that directly the breech is partly opened a blast of compressed air is allowed to enter the rear end of the chamber and thus sweep the whole of the residual gas out at the muzzle.

The purpose of the obturator is to render the breech end of the gun gas-tight, and to prevent any escape of gas past the breech mechanism. In the first Armstrong B.L. gun this object was attained by fitting to the breech block a copper ring coned on the exterior; the coned surface was forcibly pressed by Obturators. screwing up the breech screw against a corresponding copper ring fitted at the breech opening of the gun chamber. It is only possible to use this method when the copper surfaces can be jammed together by a powerful screw.

Except the above, all obturators in use are arranged to act automatically, i.e. the pressure set up in the gun when it is fired expands the arrangement and seals the opening; immediately the projectile leaves the bore the pressure is relieved and the obturator, by its elasticity, regains its original shape, so that the breech mechanism can be opened or closed with ease. In the French naval service B.L. guns have been in use since 1864, and the system of obturation was arranged on the same expansion principle as the leather packing ring of the hydraulic press. A steel ring A (fig. 47) of cupped form was fastened by a screwed plug to a thick steel plate, carried on the face of the breech screw, so that it could rotate when the breech screw was rotated in opening or closing the gun. The outer lip of the cup fitted against a slightly coned seating formed in the breech end of the gun chamber. When the gun was fired, the gas pressure expanded the cup ring and forced it into close bearing against the seating in the gun and the thick steel plate on the breech screw, thus preventing any escape of gas. Very similar to this was the Elswick cup obturator (fig. 48) introduced by the Elswick Ordnance Company in 1881; its rear surface was flat and it was held by a central bolt against the front of the breech screw which was slightly rounded. The cup yielded to the gas pressure until it was supported by the breech screw; this action expanded the lip against a copper seating, let into the gun, which could be renewed when necessary. Many of both types are still in use and act perfectly efficiently if carefully treated. The use of modern smokeless powder renders them and similar devices, such as the Broadwell ring (fig. 49), &c., peculiarly liable to damage, as a slight abrasion of the lip of the cup or ring, or of its seating, allows gas to escape, and so accentuates the defect with each round fired. Unless, therefore, the fault be immediately remedied considerable damage may be caused to the gun. The Broadwell gas ring is still in use in the French naval service, where

Fig. 43. — Breech Mechanism, Modern 6-in. Gun.

it is made of copper (fig. 50), and also of steel in a modified form (Piorkowski) in the German service (fig. 51); in the last-named service, owing to the defect already named, all the latest guns, both light and heavy, use metal cartridge cases. In the French navy, as in