Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/310

296 GUNMAKING [ORDNANCE. groove ; the rear one is the largest, the front one the smallest. They are so placed that the line of their loading sides is parallel to the angle made by the groove at the breach, while the line of their driving edges is parallel to the angle made by the groove at the muzzle. In Plate IV. fig. 1 shows their position when the shell is home, and also when about to leave the bore. Thus, theoretically, the middle and leading stud should never come into play till the last moment, and the rear stud should do all the work of rotation. Practically, however, the rear stud, being of bronze, wears down against the edge of the hard groove ; the centre stud obtains a bearing ; the wear continues, and the leading stud takes up its share of duty ; so that all three really act during the passage of the projectile through the bore. This method of employing the increasing twist necessitates wide grooves, and is unsuitable for small guns. A different plan therefore was devised to meet their case. Two rows of studs, front and rear, were so arranged that each ran in a groove of its own. The curve of the grooves Studs, belonging to the rear stud began at 0, and reached the final angle at a distance from the muzzle equal to the dis tance between the front and rear rows, after which it pro ceeded in a straight line. The curve of the grooves belong ing to the front studs began at 0, at a distance in front of the commencement of the other grooves equal to the dis tance between the rows, and reached its final angle at the muzzle. Thus the two curves were alike, but one was always a constant distance in front of the other, and every stud acted through the entire length. Fig. 2 in Plate IV. shows the position of the studs when the shot is home, and also when it is about to leave the bore. Muzzle-loading rifling had reached this stage of progress at a time when the great wear caused by the rush of gas over the shot necessitated the adoption of some method of sealing the escape. After many trials, it was found that a flanged copper disc fitted on to the base of the projectile would expand under the first pressure of the gas, and cut off the windage completely. This saved the wear of the guns, and added to their power. It was found, moreover, Gas that the flanges of these " gas checks " expanded in the checks, grooves, and being firmly attached to the shell afforded an additional means of giving rotation. The next step was to remove the studs altogether, using a smooth projectile, and substituting many small grooves for few large ones. This metho.l of giving rotation has been adopted in all recently- designed pieces, and appears likely to give entire satisfaction. When muzzle-loading guns were first rifled, many attempts were made to give rotation by expanding projectiles, but they proved unsuccessful, because at that time the increas ing twist was not known, and the action of a violent powder and the uniform twist was that the rotator had to expand into the grooves, and at the same instant to communicate a rapid spin to the shell. It was unable to combine these operations, and the projections driven out by the gas could never settle to their work, being constantly cut down by the edges of the grooves. The result was that the shell " stripped," and left the bore without regular rotation. In the case of the twist which begins at 0, the projections have time to expand comfortably, and adapt themselves to the shape of the grooves before they are called on to perform the work of rotation. On the other hand, since the gas presses out the flanges against the bore and into the grooves, and affords them support in this position, it is desirable that its pressure should correspond with their pressure on the driving edges of the grooves. This would be exactly met by the uniform twist, but that is inadmissible for the reason just given. The parabola, on the other hand, throws the principal rotating strain forward towards the muzzle, where the gas pressure is least (Plate IV.). The case is practically met by the semicubical parabola, which is the curve adopted in the 80-ton gun. While progress was thus being made in the rifling of muzzle-loaders, the rival system had not remained inactive ; the use of lead coating had been found Lead to involve a loss of power in the projectile as regarded both coatir the amount of bursting charge or bullets carried, and the penetration into armour, on to the surface of which the lead flew forward in a splash, when the shell struck. Moreover, the lead envelope would not lend itself to the exigencies of the increasing twist. To meet these difficulties, two under cut rings were machined round the smooth projectile, one near the base, the other near the shoulder. Into these slots were pressed stout copper wires, the rear one bringing up Coppi the diameter of the shell beyond that of the bore, so as to bands take the rifling when forced through the piece on discharge, the front one just fitting the lands, but not entering the grooves, so as to rotate irrespective of them, and thus keep the fore part of the shot steady. Projectiles thus fitted are more effective than the lead-coated ones, and are adapted to auy twist of rifling. The latest breech-loaders iirnde by the Elswick Company use the new arrangement described above for muzzle-loaders the smooth shell with a gas check on the base. This appears to be the best form of projectile, and has the advantage of suiting either system. A few words on the section of groove best suited to various pur poses will conclude the subject of rifling. The rotation is impressed on the projectile by a force acting tangentially to the surface. This principle would be represented in its extreme form by making the section of the shell to resemble a cog-wheel, the grooves and lands of the gun being bored to fit. With muzzle-loading systems where windage has been necessary, the sides of the projections on the projectile, and of the grooves in the bore, have been sloped to enable the projections to run up the sides of the grooves, and so distribute the windage equally all round. The extreme form of this principle is Lancaster s oval bore, the section of which departs but slightly from a circle. All attempts at "centring" the projectile by sloping the bearings tend Cent- to convert it into a wedge, which expends that part of the ril) K- force resolved radially in a crushing action on the shell, and a rending action on the gun. The slope may, however, be made of such a degree as to cause the projectile to centre fairly without exercising any important effect on the bore. In breech-loading systems, where the lands bite into the rotator, the driving edge strikes the projectile in, or nearly in, the prolongation of the radius, care being taken that the points of action are as many as possible consistently with leaving enough material between to stand the wear and the strain. Where gas checks expand in the grooves, these should be broad and flat, so that a complete fit and a strong projection should result. The driving edges should here also form a prolongation of the radius of the projectile. Conclusion. What may be the future of firearms it is ConcI impossible to predict, but it seems probable that the limit sion - of power will be found to lie in the recoil. F^r shoulder guns, methods of withstanding increased velocities may be devised, but the weight of springs, pads, <fec., will prevent the soldier or the sportsman from carrying this principle very far. For ordnance, the field artillery appear even now to be very near the limit of what power can be gained consistently with high mobility. Siege and heavy guns will no doubt gain by the increased application of hydraulics; but it is difficult to see how progress can go much further except in the matter of size, to which there appears to be practically no limit. Accuracy may be improved by moro perfect methods of range finding, but, as regards the actual shooting of the guns, it is already in advance of the difficul ties of atmospheric irregularity, which affects the smaller projectiles greatly. The reader who wishes to study the subject closely and technically is referred to the list of works

given at the end of the article on GUNNERY. (E. M.)