Oblong Projectiles are fired principally from rifled pieces, and have been adopted on account of the increase of range and accuracy which can be obtained with them. For this purpose it is necessary that the projectile should move through the air in the direction of its length. Though experience would seem to show that the only sure method of effecting this is to give it a rapid rotary motion round its axis by the grooves of the rifle, numerous trials have been and are now being made to produce the same effect with smooth-bored guns. One of the simplest plans for this purpose is to place the centre of gravity or inertia in advance of the centre of figure. Another is to make the projectile very long, with its rear portion of wood, and its point of lead or iron, somewhat after the manner of an arrow; but these plans do not seem to be of much practical utility. The system by which the desired result is obtained with the greatest certainty is the rifle system.
Rifle System.—Spiral grooves are cut into the bore of the piece, or it is ribbed with spiral bands, and the projectile is so formed or prepared as to follow them as it passes along the bore of the piece. The principal question which now occupies the attention of those engaged in improving this species of cannon is to obtain the safest and surest means of effecting this object. Various plans have been tried to obtain the proposed object; nearly all may be ranged under the following heads:
1. The Flanged System.—This comprises all projectiles which have certain flanges or projections to fit into the grooves of the gun in loading. These are usually deep and few in number, rounded at their bottom edges so as to cause the flanges or studs to pass up the inclined side when rotation is imparted. This is the system at present adopted in England. Though this plan affords a certain means of communicating the rifle motion, it has not always been found a safe one, probably from the wedging of the flanges in the grooves. Besides, the dirt from the burning of the powder collects in the grooves; and as it is difficult to clean them by the usual means, the projectile is liable to meet with obstruction in loading. To obviate these difficulties, the flanges are sometimes made of softer metal than the body of the projectile. Guns for flanged or studded projectiles usually have from 3 to 9 grooves, 0.15 to 0.25 inch deep.
2. The Compressive System.—By this system the projectile is forced by the action of the powder through the bore of a piece whose diameter without the grooves is less than the diameter of the projectile. Such are the projectiles for the breech-loading Krupp and Broadwell guns. These usually consist of cast iron or steel, and are covered with a coating of lead or other soft metal having horizontal ribs or corrugations, which is secured by a chemical solder, or cast into undercuts in the body of the shot. As the projectile is forced through the bore, an impression of the rifling is cut out of the ribs, the lead thus displaced finding room in the grooves between. This system has been found to work satisfactorily in breech-loading guns. The rifling should be shallow and consist of numerous grooves, slightly narrowing towards the muzzle. Large guns usually have from 20 to 76, from .05 to .08 inch deep. Experiments are now being made, with prospects of success, to substitute bands of soft copper encircling the projectile for the lead coating.
3. The Expansive System.—This system has been so exclusively used in the United States that it has frequently been called the American system. It embraces all projectiles which are loaded without regard to the rifling, but which are fitted with an expanding portion of some softer metal, as pewter, copper, wrought iron, or papier-maché, which is forced into the grooves by the discharge. This system requires for its rifling fewer grooves than the compressive, but a somewhat greater number than the flanged system. Among the projectiles of this class used during the civil war were the Blakely, Dyer, Hotchkiss, James, Parrott, Reed, Schenkle, and Stafford. The principal objections to an expanding or compound projectile are its want of strength to resist a charge of powder proportionately as large as that employed for a simple projectile, and the danger of its breaking and wedging in the bore of the piece. Of late years, however, marked improvements have been made, and projectiles of this class can now be safely fired with double their former charges. The large projectiles of this description now used in the United States consist of the usual cast-iron body having a sabot, or ring of brass or copper either cast or screwed to its base. This ring is divided into an upper and lower flange or lip by an annular groove. When the gun is fired, the gases enter this groove, forcing the lower flange down upon the projectile and the upper or outer into the rifling of the gun, where it is kept during its passage through the bore.
Armstrong Projectile.—But one kind of projectile is used in the Armstrong breech-loading guns for the field service, and this is so constructed as to act as a shot, shell, or case-shot at pleasure. It consists of a very thin cast-iron shell, inclosing 42 segment-shaped pieces of cast iron built up so as to form a cylindrical cavity in the centre, which contains the bursting charge and the concussion-fuze. The exterior of the shell is thinly coated with lead, which is applied by placing the shell in a mold and pouring it in a melted state. The lead is also allowed to percolate among the segments, so as to fill up the interstices, the central cavity being kept open by the insertion of a steel core. In this state the projectile is so compact that it may be fired without injury, while its resistance to a bursting charge is so small that less than one ounce of powder is required to burst it. When the projectile is to be tired as a shot, it requires no preparation; but the expediency of using it otherwise than as a shell is doubted. To make it available as a shell, the bursting tube, the concussion- and time-fuzes, are all to be inserted; the bursting tube entering first and the time-fuze being screwed in at the apex. If the time-fuze be correctly adjusted, the shell will burst when it reaches within a few yards of the object; or failing in this, it will burst by the concussion-fuze when it strikes the object, or grazes the ground near it. If it be required to act as a canister-shot upon an enemy close to the gun, the regulation of the time-fuze must be turned to the zero of the scale, and then the shell will burst on leaving the gun. The Armstrong projectiles for the muzzle-loading guns have rows of brass or copper studs projecting from their sides to tit into the grooves of the gun, which are constructed on the [shunt] principle. The projectile is made of wrought iron, or low steel, with very thick sides. There is no fuze, the explosion resulting from the heat generated by the impact, and the crushing in of the thin cap which closes the mouth of the powder-chamber. The sides and bottom of the shell being thick enough to resist crushing by the impact, and also to resist the explosive force of the bursting charge, its effect will, after penetration, be expended on the backing of the armor, or the decks which the armor is intended to screen. Such projectiles are called “blind shells.”
Blakely Projectile.—Capt. Blakely’s projectile has an expanding cap attached to its base by means of a single tap-bolt in the centre. It is prevented from turning by radial grooves cast on the surface of the bottom of the projectile, into which the cup is pressed by the charge. The angle between the curved sides of the cup and the bottom of the projectile is filled with a lubricating material. On the forward part of the body are soft metal studs, more numerous than the grooves of the bore of the piece, that some of them may always form a bearing surface for the projectile against the lands. The driving sides of the grooves are deeper than the others.
Dyer Projectile.—The Dyer projectile is composed of a cast-iron body, and a soft metal expanding cup, attached to its base. The adhesion of the cup is effected by tinning the bottom of the projectile, and then casting the cup on to it. The cup is composed of an alloy of lead, tin, and copper, in certain proportions. This projectile, as improved by Mr. Taylor at the Washington Arsenal, gives good results for even as large a caliber as 12 inches.
French Projectile.—The projectile used in the French field service is made of cast iron, and has 12 zinc studs on its sides, arranged in pairs, so as to fit the 6 grooves of the gun. For the larger cannon projectiles, but 3 studs are used, and these are cast on the projectile, nearly opposite to its centre of gravity; the bearing sides of the studs are faced with white metal to diminish friction against the grooves of the bore. The shape of the grooves is such as to centre the projectile. The latter projectile is used with increasing, the former with grooves of uniform twist. Russian, Austrian, and Spanish artillery projectiles belong to the studded, or button class, but differ from each other in the details of their construction.
Hotchkiss Projectile.—The Hotchkiss projectile is composed of three parts: the body, the expanding ring of lead, and the cast-iron cup. The action of the charge is to crowd the cup against the soft metal ring, thereby expanding it into the rifling of the gun. The time-fuze projectile has deep longitudinal grooves cut on its sides to allow the flame to pass over and ignite the fuze. The last rifle projectile submitted by Mr. Hotchkiss has an expanding cup of brass attached to its base in a peculiar manner. The cup is divided into four parts by thin projections on the base of the projectile. This arrangement is intended to facilitate the expansion of the cup and to allow the flame to pass over to ignite the fuze.