which was simply pushed into the vent and fired by the slow
match or port-fire.
Table I.
| Length of projectile in calibres. | Minimum twist at muzzle of gun requisite to give stability of rotation = one turn in n calibres; or a pitch of n calibres. | |||
| Cast-iron common shell, cavity=827, vol. of shell (s.g. of iron=7·2). | Palliser shell, cavity = 18 vol. (s.g. =8·0). | Solid steel bullet (s.g.=8·0). | Solid lead and tin bullets of similar composition to M.-H. bullets (s.g.=10·9) | |
| n. | n. | n. | n. | |
| 2·0 | 63·87 | 71·08 | 72·21 | 84·29 |
| ·1 | 59·84 | 66·59 | 67·66 | 78·98 |
| ·2 | 56·31 | 62·67 | 63·67 | 74·32 |
| ·3 | 53·19 | 59·19 | 60·14 | 70·20 |
| ·4 | 50·41 | 56·10 | 57·00 | 66·53 |
| ·5 | 47·91 | 53·32 | 54·17 | 63·24 |
| ·6 | 45·65 | 50·81 | 51·62 | 60·26 |
| ·7 | 43·61 | 48·53 | 49·30 | 57·55 |
| ·8 | 41·74 | 46·45 | 47·19 | 55·09 |
| ·9 | 40·02 | 44·54 | 45·25 | 52·72 |
| 3·0 | 38·45 | 42·79 | 43·47 | 50·74 |
| ·1 | 36·99 | 41·16 | 41·82 | 48·82 |
| ·2 | 35·64 | 39·66 | 40·30 | 47·04 |
| ·3 | 34·39 | 38·27 | 38·84 | 45·38 |
| ·4 | 33·22 | 36·97 | 37·56 | 43·84 |
| ·5 | 32·13 | 35·75 | 36·33 | 42·40 |
| ·6 | 31·11 | 34·62 | 35·17 | 41·05 |
| ·7 | 30·15 | 33·55 | 34·09 | 39·79 |
| ·8 | 29·25 | 32·55 | 33·07 | 38·61 |
| ·9 | 28·40 | 31·61 | 32·11 | 37·48 |
| 4·0 | 27·60 | 30·72 | 31·21 | 36·43 |
| ·1 | 26·85 | 29·88 | 30·36 | 35·43 |
| ·2 | 26·13 | 29·08 | 29·55 | 34·49 |
| ·3 | 25·45 | 28·33 | 28·78 | 33·59 |
| ·4 | 24·81 | 27·61 | 28·05 | 32·74 |
| ·5 | 24·20 | 26·93 | 27·36 | 31·94 |
| ·6 | 23·65 | 26·32 | 26·74 | 31·21 |
| ·7 | 23·06 | 25·66 | 26·08 | 30·44 |
| ·8 | 22·53 | 25·08 | 25·48 | 29·74 |
| ·9 | 22·03 | 24·51 | 24·91 | 29·07 |
| 5·0 | 2156 | 23·98 | 24·36 | 28·44 |
| ·1 | 21·08 | 23·46 | 23·84 | 27·83 |
| ·2 | 20·64 | 22·97 | 23·34 | 27·24 |
| ·3 | 20·22 | 22·50 | 22·86 | 26·68 |
| ·4 | 19·81 | 22·05 | 22·40 | 26·14 |
| ·5 | 19·42 | 21·61 | 21·96 | 25·63 |
| ·6 | 19·04 | 21·19 | 21·53 | 25·13 |
| ·7 | 18·68 | 20·79 | 21·12 | 24·66 |
| ·8 | 18·33 | 20·40 | 20·73 | 24·20 |
| ·9 | 18·00 | 20·03 | 20·35 | 23·75 |
| 6·0 | 17·67 | 19·67 | 19·98 | 23·33 |
| 7·0 | 14·99 | 16·68 | 16·95 | 19·78 |
| 8·0 | 13·02 | 14·48 | 14·72 | 17·18 |
| 9·0 | 11·50 | 12·80 | 13·00 | 15·18 |
| 10·0 | 10·31 | 11·47 | 11·65 | 13·60 |
The first attempt to fire guns by mechanical means was made in 1781 by Sir Charles Douglas, who fitted flint locks, similar to musket locks, but with the trigger actuated by a lanyard, to the guns on board his ship H.M.S. “Duke.” A double flint lock introduced in 1818 by Sir Howard Douglas, R.A., continued to be used until about 1842, when it was replaced by a percussion lock invented by an American named Hiddens. In this lock one pull on the lanyard caused the hammer to fall and strike a percussion patch or cap hung on a small hook over the vent, and afterwards caused the hammer to be drawn backwards out of the way of the blast from the vent. These somewhat clumsy contrivances were swept away on the adoption in 1853 of friction tubes (see Ammunition), which had simply to be placed in the vent and the friction bar withdrawn by means of a lanyard.
Friction tubes continued to be used with all muzzle-loading ordnance except in one or two natures with which the charge was ignited axially at the breech of the gun. In these a vent sealing friction tube retained in the vent by a tube holder was employed. With breech-loading field guns ordinary friction tubes were also used until the introduction of cordite, which eroded the vents so quickly by the escape of the gases that vent seaHng tubes became a necessity.
In all other breech-loading ordnance and with the latest
pattern field guns the firing gear forms part of the breech mechanism.
All modern breech mechanisms form two groups (a) the sliding type as with the Krupp wedge system, (b) the swinging type as in the interrupted screw system. Either type may be used with B.L. guns (i.e. those with which the charge is not contained in a metallic cartridge case)Breech mechanism. and Q.F. guns (i.e. those with which a metallic cartridge case is used).
Sliding mechanisms may be divided into two forms: (1) those having the block or wedge sliding horizontally, and (2) those in which the block works in a vertical direction, (1) is that used principally by Krupp; (2) is best illustrated by the Hotchkiss system for small Q.F. guns; the Nordenfelt, Skoda and the Driggs-Schroeder mechanisms for small Q.F. guns are an adaptation of the same principle.
The Krupp gear is in reality an improved Cavalli mechanism; it is capable of being worked rapidly, is simple, with strong parts not liable to derangement, except perhaps the obturator. The breech end of the gun, however, occupies valuable space especially when these guns are mounted in the restricted turrets or gun houses on board ship.
Later it will be seen that owing to the difficulty of arranging a convenient and efficient obdurating device for the smokeless nitropowders, which have a peculiarly severe, searching effect, a metal cartridge case has to be used with even the heaviest guns; naturally this assumes large dimensions for the 305 m/m. gun.
The wedge (fig. 32) is housed in the breech piece, which covers the breech part of the barrel, made very massive and extended to the rear of the barrel. A slot, cut transversely through the extended portion, forms a seat for the sliding block. The slot is formed so that its front is a plane surface perpendicular to the axis of the gun, while the rear is rounded and slightly inclined to the axis. One or more ribs similarly inclined on the upper and lower surfaces of the slot guide the breech block in its movements. For traversing the block a quick pitched screw is fitted to its upper surface and works in a nut attached to the upper part of the slot (in small guns this traversing screw is dispensed with, as the block can be easily moved by hand). As the rear seat of the sliding block is inclined, there is a tendency for the block to be moved sideways, when the gun is fired by the pressure in the chamber acting on the front face of the wedge; this is prevented by a locking gear, consisting of a cylinder, having a series of interrupted collars, which is mounted on a screw. When the breech has been traversed into position, the collars are rotated, by a cross handle at the side of the block, into grooves cut in the rear surface of the slot; a further movement makes the screw jam the collars hard in contact with the gun and secures the breech. With small guns having no traversing gear a short strong screw takes the place of the collars, and on the handle being turned enters a threaded portion at the rear surface of the slot, actuates the breech for the last (or first in opening) portion of its movement in closing and secures it. To open the gun the movements are reversed.
The gun is fired by a friction tube, screwed into an axial vent bored through the sliding block, or, in field guns, by a copper friction tube through an oblique vent drilled through the top of the breech end of the gun and through the block.
There is also fitted in some guns a percussion arrangement for firing a percussion tube.
The obturation is effected by a Broadwell ring or some modification of it; this is placed in a recess cut in the gun and rests against a hard steel plate fitted in the breech block.
For modern Krupp mechanisms, for use with cartridge cases, the arrangement (fig. 33) is very similar to that described above, but some improvements have added to its simplicity. The transporting screw is fitted with a strong projection which, at the end of the movement for closing the breech, locks with a recess cut in the upper surface of the slot and secures the breech. The extra locking device is consequently dispensed with. The firing gear consists of a striker fitted in the sliding block in line with the axis of the gun; the striker is pushed back by a lever contained in the block and, on release, is driven forward against the primer of the cartridge case by a spiral spring.
In the Hotchkiss gun the mechanism has a vertical breech block of a rectangular section. The actuating lever F (fig. 34) is on the right side of the gun, and connected to a powerful crank arm C working in a groove E cut on the right side of the breech block. By pulling the lever towards the rear, the crank arm forces down the block A and extracts the fired case by an extractor X, which is actuated by a cam groove Y cut on one side or on both sides of the block. As the mechanism is opened the hammer H is cocked ready for the next round. To close the mechanism the lever is pushed over to the front, and by releasing the trigger sear by pulling the lanyard the hammer falls and fires the cap of the cartridge case.
Automatic gear is now generally fitted which opens the breech as the gun runs up after recoil and extracts the fired case by means of a supplementary mechanism and strong spring actuated by the recoil of the gun, and on pushing a new cartridge into the gun the breech which was retained by the extractor is released and closes automatically.
