but although this system had certain advantages it did not fulfil all requirements.
In 1863, England re-opened the whole question, and after exhaustive trials of various inventions decided on the adoption of the muzzle-loading type for all guns, with the French system of rifling. This system was invented in 1842 by Colonel Treüille de Beaulieu and consisted of a few wide and deep grooves which gave rotation to a studded projectile. At the first trials two grooves only were tried, but the number was afterwards increased to three or more, as it was found that two grooves only would not correctly centre the projectile. The adoption of the muzzle-loading system with studded shot was a distinctly retrograde step, as a considerable amount of clearance was necessary between the bore and projectile for the purposes of loading, and this resulted in the barrel being seriously eroded by the rush of gas over the shot, and also led to a considerable loss of energy. In the Wahrendorff and Armstrong systems however the lead-coated projectiles entirely prevented windage, besides which the projectile was perfectly centred and a high degree of accuracy was obtained.
Shunt rifling was a brief attempt to make loading by the muzzle easy without forfeiting the centring principle: in this the rifling varied in width and in depth, at different portions of the bore in such a manner that, during loading, the studs on the projectile could move freely in the bore. When the gun was fired the studs of the projectile were forced to travel in the shallow part of the rifling, thus gripping and centring the projectile as it left the muzzle.
With uniform rifling on the French system, the few studs—generally two per groove—had to bear so high a pressure to produce rotation that they sometimes gave way. This subject was investigated by Captain (Sir Andrew) Noble, who showed that by making the rifling an increasing twist, commencing with no twist and gradually increasing until the necessary pitch was obtained, the maximum pressure due to rotation was much reduced. Increasing rifling was consequently adopted, with beneficial results.
In order to prevent the heavy erosion due to windage, a gas check was adopted which was attached to the base end of the studded projectiles. In some guns the number of grooves of the rifling was sufficiently great to admit of rotation being insured by means of the gas check alone; in these guns studded projectiles were not employed, but the gas check, called “automatic,” to distinguish it from that fitted to studded projectiles was usually indented around its circumference to correspond with the rifling of the gun. It was found that the studless projectile had considerably greater range and accuracy than the studded projectile, with the additional advantage that the shell was not weakened by the stud holes.
The introduction of the plain copper driving band for rotating projectiles with breech-loading guns included a return to the poly groove system with shallow grooves; this still exists, but the continuous demand for greater power has had the effect of increasing the number of grooves from that at first considered necessary, in order to keep the rotating pressure on the driving band within practical limits.
Many ingenious devices for giving rotation and preventing the escape of gas past the projectile were tried in the early days of modern rifling. Experiments of this nature still continue to be made with a view to improving the shooting and to prevent the erosion of the bore of the gun. Briefly considered, without going into any detail of the numerous plans, all rotating devices fitted to projectiles can be divided into three classes—the “centring,” the “compressing” and the “expansion” systems. The two last named almost invariably include the “centring” type. Studded (fig. 23) and Whitworth (fig. 24) hexagonal projectiles, which can freely slide in the bore, come under the first system.
In the compression class the coating or rings on the projectile are larger in diameter than the bore and when fired the coating (or rings) is squeezed or engraved by the rifling to fit the borethe projectile is consequently also centred. The old-fashioned lead-coated shell (fig. 25), and the modern system of plain copper driving bands (fig. 26), come under this class. Most variety exists in the expansion type, where the pressure of the powder gas acts on the base of the projectile or on the driving ring and compresses a lead, copper or asbestos ring into the rifling grooves. One of the earliest was the Hotchkiss (1865) shell (fig. 27), in which a separate base end B was driven forward by the gas pressure and squeezed out the lead ring L into the rifling. The automatic gas check (fig. 28), and the gas check driving band (fig. 29), belong to this system; in the last the lip L is expanded into the rifling groove. In fig. 30 a copper driving band is associated with an asbestos packing A, contained in a canvas bag or copper casing made in the form of a ring on the principle of the de Bange obturator; but the results of this have not been entirely satisfactory.
Figs. 23-30.—Projectiles for Rifled Ordnance.
It win be seen that with breech-loading guns the projectile is better centred, and the copper driving band forms a definite stop for the projectile; and, in consequence, the capacity of the gun chamber is practically constant. In addition, the use of a copper driving band ensures a uniform resistance while this is being engraved and the projectile forced through the gun, and also prevents the escape of gas. These elements have a very great influence on the accuracy of the shooting, and fully account for the vastly superior results obtained from breech-loading ordnance when compared with the muzzle-loading type. Driving bands of other materials such as cupro-nickel and ferro-nickel have also been tried.
Many authorities believe that the best results are obtained when the projectile is fitted with two bands, one near the head and the other near the base, and no doubt it is better centred when so arranged, but such shot can only be fired from guns rifled with a uniform twist, and it must also not be forgotten that the groove formed for the front band in the head of the projectile necessarily weakens that part of the projectile which should be strongest.
Projectiles with a driving band at the base only can be fired from guns rifled either uniformly or with increasing twist.
The introduction of cordite (q.v.) about 1890 again brought into special prominence the question of rifling. The erosion caused by this explosive soon obliterated the rifling for some 4 or 5 calibres at the breech end. The driving band of the shell consequently started with indifferent engraving, and with the increasing twist, then in general use, it was feared that the wear would quickly render the gun useless. To remedy this the late Commander Younghusband, R.N., proposed straight rifling, which was adopted in 1895, for that portion of the rifling mostly affected by the erosion, with a gradual increase of the twist thence to the required pitch at the muzzle. Thus, any erosion of the straight part of the rifling would not affect that portion giving rotation, and it was argued that the gun would remain efficient for a longer period. The defect in this system is that when the projectile arrives at the end of the straight rifling it has a considerable forward velocity and no rotation. Rotation is then imparted by the increasing twist of rifling, and the
