The different systems of projectiles for rifled pieces are classified according to the man ner in which they are caused to follow the grooves. The systems are distinguished by some peculiarity of construction and are gen erally known by the name of the person by whom designed or by the place at which first made. The same principles are applicable to different systems, and the same precision of fire may practically be obtained from several, All systems are comprised under three classes: (1) Projectiles with ribs or studs, or having a peculiar cross-section. (2) Pro jectiles having a portion that is expanded by the action of the gasses in the bore. (3) Those more or less of whose surface is compressed by the charge into the form. The first and second classes are applicable to muzzle-loading pieces; the third to breech-loaders only. The principal points to be considered, as regards the piece and the projectile, are the surest and safest means of causing the projectile to follow the grooves of the piece. To fulfill these condi tions the projectile should be simple in construc tion and of sufficient strength to admit of its use with the largest charge that may be desired; it must not be liable to jam in the bore in load ing or firing, and must produce a moderate and uniform strain on the gun. The system that most nearly complies with these requirements, and gives insured accuracy of fire with uni form and high velocities, should be the best. In many systems, one or more of these consid erations have been sacrificed to some extent, to secure a closer compliance with others thought to be of greater importance or of easier at tainment.
1st Class.— Solid flanges, projecting from the body of a projectile, and so shaped as to fit the rifling of the bore, were the means first used to communicate the rifle-motion in cannon. In some cases there was for each groove a rib extending the entire length of the cylindrical portion of the projectile, while in others sets of rounded buttons were employed. These projec tions were of the same material as the body of the projectile, and, being of a very unyielding nature, frequently led to the bursting of the piece; buttons of zinc, copper or bronze, firmly secured in mortises in the projectiles, were therefore adopted. The buttons are arranged in rows or two or more so that each row enters freely into a corresponding groove, in loading. When the bore of a gun is a twisted prism, with any plane figure for its base, the projec tile, if shaped to it, will receive the rifle-motion when fired. The Whitworth cannon is rifled in this manner, the cross-section being a hexa rounded corners. Guns have also rn been constructed with ribs projecting from the bore, fitting into corresponding grooves in the projectile. To the last system belongs the
Vavasseur gun, which has given good results. The principal advantages of systems of the first class are that the projectiles are strong and that the required motion is communicated to them with great certainty and regularity. The escape of gas around the projectile, as it causes injury to the bore, is an objection to the class; various experiments have been made to over come it, one being by the attachment of a metal to the rear of the projectile, which, by the action of the powder, is expanded and presses against the sides of the bore. The French studded projectile, employed with muzzle-load ing cannon, and the Woolwich system, similar to it are the best representatives of this class.
2d Class.— In projectiles of the second class the body is composed of a hard metal, as cast iron, and there is attached to it, generally at the base, a cup, band or other arrangement of softer metal, which is expanded by the action of the charge into the grooves of the gun, when fired. Expanding projectiles are easy to load, are not liable to overstrain the piece, and those of different systems can generally be fired from the same piece — a point of great importance. Such projectiles do not always withstand the heaviest charges of powder, and are not certain to receive the rifle motion. The use of projec tiles of this class is confined more particularly to the United States.
3d Class.— In breech-loading cannon the receptacle for the charge is of a larger diame ter than the bore of the piece. The projectile is of the same size as this chamber, and must be reduced to enable it to pass into the bore. Such projectiles are embraced under the third class; the body has a coating of soft metal, which is compressed as the projectile is driven through the bore, the grooves compelling it to follow the direction of the rifling. The same result is sometimes accomplished by one or more rings of soft metal. The chief advan tages of this class are that the projectile is gen to take up the rifle-motion; that its axis is steady on leaving the bore, and that the wear of the bore, from gas passing the pro jectile or from any irregular movement of the latter, is prevented. The objections are that the necessary compression of the coating, the sud den closing of windage and the fouling of the bore produce undue strain upon the piece; that the velocity of the projectile is reduced by the force expended in compressing it; and that the soft metal forms an extra weight, which is useless in penetrating resisting objects.