" Scoring," once it begins, increases very rapidly and grows at an increasing rate as firing progresses. A tool-mark in the vicinity of the seat of the projectile is likely to start scoring; but its importance is lessened the further it is from that point. Scoring is chiefly found on the front incline of the chamber, and thence for a few feet down the bore. So long as it continues to increase only gradually in proportion to the number of rounds fired it may be considered normal and of no great consequence while it remains within certain limits.
In addition to the eventual unserviceability of the gun, the effect of wear and scoring is shown during the life of the gun by a gradual falling-off of muzzle-velocity and accuracy.
During the World War, in which ammunition expenditure far exceeded what was foreseen, several new expedients were used to augment the life of the guns. To this requirement is largely due the lengthening of howitzers, the increased use of reduced charges, and the development of new projectile forms which in comparison with the old gave equal results with and smaller strain on the gun, or brought a given target within the range of a smaller (and longer-lived) gun.
Apart from these indirect methods, various expedients were applied to the gun itself, notably by the French. For instance, in the 145 new heavy field gun provision was made in the design for boring out to 155 mm. after a first life at the normal calibre, and the 155 G.P.F. when worn had its chamber lengthened, the rearmost 12 in. of rifling being removed. (X.)
(IV.) BUFFER AND RECUPERATION DESIGN
On firing the recoiling gun acquires kinetic energy which must be absorbed by a resistance acting over the recoil permitted ; this resistance is provided by the buffer, recuperator and friction. To reduce the recoil velocity and energy, particularly with field carriages, the heavier portion of the recoil system is generally made to recoil with the gun, the lighter portion being fixed to the cradle. The recuperator returns the gun to the firing position after it has come to rest under the action of the recoil resistance. It must be designed with a reserve of power, and so it assists in checking recoil. 1 The buffer must absorb the energy not accounted for by the recuperator and friction, and must adjust the total resistance as required by the carriage. With field carriages stability must be kept during its action.
The buffer consists of a cylinder filled with oil in which works a piston and rod; either the cylinder or, more usually, the rod is fixed, the remaining portion recoiling with the gun. A flow- space for the liquid, arranged to decrease during recoil, is provided between the piston and cylinder. The hydraulic resistance of the liquid to being forced through the flow-space causes pressure on the face of the piston and a pull on the rod. The greater the flow-space the less is the pull on the rod, so that . by a suitable arrangement the resistance can be varied.
Modern buffers are tension buffers, the piston-rod being withdrawn during recoil. A compression buffer, in which the rod is pushed in, has the disadvantage that the free space in the cylinder cannot be fully occupied by liquid, as room must be kept for the volume of the rod when it enters. One com- pression buffer, however, is in use, in which the rod is extended beyond the piston and passes out of the cylinders through pack- ing, thus permitting the cylinder to be completely filled.
At full recoil, a length of the piston-rod is withdrawn leaving a partial vacuum; on recuperation the liquid must be banked up against the face of the piston remote from the rod before the buffer resistance can take effect. At this instant the gun has its maximum recuperative velocity, and the larger the ratio of diameter of piston to rod the sooner will the buffer resistance act as a control. The flow-space in the buffer increases during recuperation, and the resistance is not sufficient to absorb the
' In certain modern equipments the energy stored in the recupera- tor during recoil is used not only to run the gun up but also to actuate the mechanism which unlocks the breech-mechanism of the gun holds it back while the gun runs forward, and then, after reloading is completed brings the breech-block up to and into the breech opening and locks it. A French example is given above. (C. F. A )
surplus energy. A further controlling device is therefore provided to ensure that the gun comes quietly to rest in the firing position.
The stability of a field carriage increases with the elevation, and at the horizontal it demands that RXH must be less than WXL w r XX.
Further, not only must the carriage be steady, but it must have a definite stability-margin throughout recoil; thus, if N represents the percentage increase in R to reach the stability limit
RXH=WXL-w r XX
or 100 WXL-w r XX
ioo+N H
throughout recoil. The stability-margin of N per cent should be at least 25, and in modern carriages approximates to 50.
Methods of Control. From these conditions it is seen that the resistance to recoil must decrease uniformly with recoil, and hence the system of graduated resistance is employed with field carriages. While the gas is acting on the gun, the recoil should be as unrestricted as possible, in order to preserve the aim of the gun and to reduce the couple on the recoiling parts. Frequently the maximum buffer resistance does not come into action until the gun has moved from 10 to 15% 6f the total length of recoil. A typical resistance-space diagram for graduated recoil is shown in fig. 2.
Recuperator
Friction
Recoil FIG. 2. Resistance-space diagram. Graduated recoil.
The condition to determine the length of recoil is that the maximum recoil energy with free recoil (which can be calculated from the ballistics of the gun) must be equal to the work done by the total resistance to recoil, that is, to the area of the resistance-space diagram. Thus the working length of recoil, which for a light field carriage varies from 40 to 45 in., can be determined; and the buffer resistance can be obtained from the diagram, for the recuperative and frictional resistance can be calculated with reasonable accuracy.
The system of " controlled " resistance shortening the recoil as elevation increases is employed with many modern field guns and howitzers on account of firing at high elevations, as there is not space to permit of the same length of recoil as at the horizontal. Since stability increases with the elevation, it is possible to increase the buffer resistance with the elevation and thus shorten the recoil. A typical resistance-space diagram for this system is shown in fig. 3. Comparing fig. 3 with fig. 2 it can be seen how the proportion of energy absorbed by the buffer increases greatly with the elevation.
With a fixed mounting the question of stability does not arise. Consequently the recoil is fixed at from 3 to 4 cals. depending on the working stress on the mounting. As the recoil must not be exceeded at the greatest elevation, the component of the recoiling weight must be allowed for. The design must be such as to absorb the energy at the greatest elevation. Further, the total resistance will be a minimum when maintained constant through- out recoil. " Equalized " resistance to recoil is adopted with fixed mountings. A typical resistance-space diagram for this is shown in fig. 4.
Buffer Dimensions. The buffer dimensions depend on the follow- ing considerations. The length of the buffer must allow of a metal- to-metal recoil slightly greater than the working length determined.
