Parrott Gun.—The Parrott rifled gun is a cast-iron piece of about the usual dimensions, strengthened by shrinking a coiled band or barrel of wrought iron over that portion of the reinforce which surrounds the charge. The body of the larger Parrott guns are cast hollow, and cooled from the interior on the Rodman plan. The barrel is formed by bending a rectangular bar of wrought iron spirally around a mandrel, and then welding the mass together by hammering it in a strong cast-iron cylinder, or tube. In bending the bar, the outer side being more elongated than the inner one, is diminished in thickness, giving the cross-section of the bar a wedge shape, which possesses the advantage of allowing the cinders to escape through the opening, thereby securing a more perfect weld. The barrel is shrunk on by the aid of heat, and for this purpose the reinforce of the gun is carefully turned to a cylindrical shape, and about one-sixteenth of an inch to the foot larger than the interior diameter of the barrel in a cold state. To prevent the cast iron from expanding when the barrel is slipped on to its place, a stream of cold water is allowed to run through the bore. At the same time, and while the band hangs loosely upon it, the body of the gun is rotated around its axis to render the cooling uniform over the whole surface of the barrel. The proof of the Parrott guns consists in firing each piece 10 rounds with service charges.
Rodman Gun.—The principal difficulty formerly experienced in manufacturing very large cast-iron cannon was the injurious strain produced by cooling the casting from the exterior. Gen. Rodman of the U. S. Ordnance Department developed a theory of the strains produced by cooling a casting like that of a cannon (see [Ordnance, Strains upon]), and as a remedy for them proposed that cannon should be cast on a hollow core and cooled by a stream of water or air passing through it. This new mode of casting was afterwards adopted by the War Department. By this system of casting, guns of greatly increased size and endurance are fabricated. The largest guns employed in the U. S. service (20-inch) are made on the Rodman plan, as well as many of the guns employed in the field service.
Whitworth Gun.—These guns are made of a species of low steel; the smaller are forged solid, the larger are built up with coils or hoops; the hoops are forced on by hydraulic pressure, and for this purpose are made with a slight taper and with the design to secure initial tension. The ends of the hoops are joined by screw-threads. The hoops are first cast hollow, and then hammered out over a steel mandrel. Before receiving their final finish they are subject to an annealing for some three or four weeks, which makes the metal very ductile, but at the same time slightly impairs its tenacity. The system differs from Krupp’s in the smaller masses used and the greater number of hoops. The process for making the hoops is better calculated to develop their tensile strength. The breech-pin is made with offsets in such a way as to screw into the end of the barrel and the next two surrounding hoops. The cross-section of the bore of the Whitworth gun is a hexagon with rounded corners. The twist is very rapid and the projectiles are made very long.
Woodbridge Gun (invented by Dr. Woodbridge, of Little Falls, New York).—The system of construction consists essentially of a thin steel barrel over which wire is wound, barrel and wire being subsequently consolidated into one mass by a brazing solder melted and poured into the interstices. The following brief description is extracted from one of the inventor’s letters to the chief of ordnance: “Square wire is wound upon a steel core somewhat longer than the intended bore of the gun, a sufficient number of wires being wound at once side by side to produce the required obliquity of the turns. The successive layers have opposite twists. When the mass has reached the required dimensions, it is inclosed in an air-tight case to protect it from oxidation, and is heated therein to a temperature somewhat above that required for the fusion of the soldering metal. The soldering metal having been melted is run in, filling all the interstices of the mass. When cooled the gun is bored and finished as usual.” The invention dates back to about 1850. A small gun made in this way was tested by Maj. Laidley (U. S. Ordnance Corps) in 1865. It endured 1327 rounds with excessive charges, when the attempt to burst it was abandoned on account of the breaking off of the trunnions. The only large gun ever made—a 10-inch gun—was fabricated at Frankford Arsenal. It was not entirely finished till April, 1876, soon after which it was displayed at the Centennial Exhibition in Philadelphia. Certain defects in its manufacture prevent it from fairly representing the Woodbridge system.
Woolwich Gun.—The Woolwich or Fraser gun is in its construction a modification of the Armstrong plan, which latter had been previously used in Great Britain; the principal difference is in substituting for a number of single coils and a forged breech-piece a few long double and triple coils, and in using a cheaper quality of wrought iron. The number of pieces employed in the construction depends upon the size of the gun; an 8-inch rifled gun is composed of the inner tube (barrel) of steel, the muzzle-coil (trousers), the breech-coil (jacket), and the cascabel-screw. The barrel is made from a solid forged cylinder of cast steel, drawn by heating and hammering; it is turned, bored, and chambered; then heated to a uniform temperature in a vertical furnace and plunged into a covered tank of rape-oil, where it cools and soaks. The muzzle-coil is constructed of two single coils welded together endways. Each coil is formed by heating a long bar and wrapping it about a mandrel; this is next heated in a reverberatory furnace and welded under a steam-hammer. Before being united the two cylinders are turned and bored. The breech-coil is composed of a triple coil, a trunnion-ring, and a double coil welded together. The double coil is formed by placing a single coil, when cold, on a mandrel and winding over it, but in the reverse directions to break joints, a second bar; if over this a third bar is immediately wound in the same direction as the first, a triple coil will result. These coils are welded by being heated and hammered on the end and on the sides. The trunnion-ring is made by welding slabs of iron together on the flat end of a bar, and gradually forming a ring by driving through the centre wedges and mandrels increasing in size; the trunnions, one of which comes from the bar, are at the same time hammered into shape. The coils and the ring having been turned and bored, the latter is placed on a shoulder of the triple coil, the double coil is dropped through the trunnion-ring on the triple coil, and the joints welded in this position. The cascabel is forged of good scrap-iron; the different parts having been formed are accurately turned and bored with a slight taper. The muzzle-coil tube being heated is dropped over the barrel, which is stood in a pit, a stream of cold water circulating through the bore. The half-formed gun is then placed on its muzzle, water forced through the bore, and the breech-coil heated and slipped into position. The cascabel is screwed into the breech-coil abutting against the barrel, great care being taken that the contact is perfect. A tell-tale hole is cut along the thread on the cascabel to give warning by the escape of gas should the barrel break in firing. The vent is bored through hardened copper; it enters near the centre of the service cartridge. This gives greater velocity, but also greater pressure. The large guns have from seven to ten grooves. The twist is uniformly increasing; the shape of the grooves is circular, with curved edges.
Sutcliffe Gun.—This invention, by E. A. Sutcliffe of New York City, relates to a breech-mechanism for cannon. See [Breech Mechanism].
Griffin Gun.—Name sometimes given to the 3-inch rifled field-piece in the U. S. service. It is made of wrought iron. The method of fabrication is to wrap boiler-plate around a mandrel and to weld it.
Ordnance, Metals for. The only metals ordinarily used for cannon are cast and wrought iron, steel, and an alloy of copper and tin, or a combination of these metals. Cannon metals should be able to resist the corroding action of the atmosphere, the heat and the products of combustion of the powder; should be susceptible of being easily bored and turned, and should not be too costly. The qualities necessary in cannon metals are strength to resist the explosion of the charge, weight to overcome severe recoil, and hardness to endure the bounding of the projectile along the bore. The shape of the bore would otherwise be rapidly altered by the action of the projectile. This quality is particularly necessary in rifled cannon. The term strength as applied to cannon metal is not confined to tensile strength alone, but embraces also elasticity, ductility, and crystalline structure, which affect its power to resist the enormous and oft-repeated force of gunpowder. (See [Ordnance, Strains upon].) Each discharge of a cannon, however small, impairs its strength, and repeated a sufficient number of times, will burst it; this arises from the fact that the feeblest strains produce a permanent elongation or compression of iron; this is technically known as the permanent set, and the same is probably true of all other metals. The property of ductility is of importance in enabling a metal to resist rupture after it has passed its elastic limit. The size and arrangement of the crystals of a metal have an important influence in its strength to resist a particular force. A metal will be strongest when its crystals are small, and the principal faces parallel to the straining force, if it be one of extension, and perpendicular to it, if it be one of compression. The size of the crystals of a particular metal depends on the rate of cooling; the most rapid cooling giving the smallest crystals.
Cast iron is very generally employed, notably in the United States, in the fabrication of heavy cannon for siege and sea-coast purposes. It possesses the very important qualities of tenacity, hardness, and cheapness, and with proper care is not seriously affected by rust. Its principal defect is an almost entire want of elasticity, in consequence of which its tenacity is destroyed after a certain number of applications of the straining force. But little is known of the causes which affect the quality of the cast iron used for cannon metal. The amount of carbon, the state of its combination, together with the ore, fuel, and fluxes, and the process of manufacture, all materially affect the quality of the iron. All that is known is, that certain ores treated in a certain way make cast iron suitable for cannon, and the fitness of a particular kind of cast iron for artillery purposes can only be determined by submitting it to the tests of the service. After this is known, a knowledge of certain physical properties, such as tenacity, hardness, density, and color, form and size of crystals presented in a freshly fractured surface, will be useful in keeping the metal up to the required standard. The pig-iron from which cannon are made should be soft, yielding easily to the file and chisel; the appearance of the fracture should be uniform, with a brilliant aspect, dark gray color, and medium-sized crystals. When remelted and cast into cannon, it should have about sufficient hardness to resist the file and chisel, but not to be so hard as to be bored and turned with much difficulty; its color should be a bright gray, crystals small, structure uniform, close, and compact. The density of gun metal should be about 7.25, and its tenacity about 30,000. There are several varieties of cast iron differing from each other by almost insensible shades. The principal divisions are, however, gray and white. Gray iron is softer and less brittle than the white, is slightly malleable and flexible, and does not resist the file. It has a brilliant fracture of a gray or bluish-gray color. This iron melts at a lower temperature than white iron and becomes more fluid, contracts less and contains fewer cavities; it fills the mold well, the edges of a casting are short, and the surface smooth, convex, and covered with carburet of iron. Gray iron is the only kind suitable for making castings which require great strength, such as cannon. White iron is very brittle, resists the file and chisel, and is susceptible of high polish, the surface of a casting is concave, the fracture presents a silvery appearance. Its qualities are the reverse of those of gray iron; it is therefore unsuitable for ordnance purposes. Mottled iron is a mixture of white and gray; it has a spotted appearance, and flows well. The casting has a plane surface with edges slightly rounded. It is suitable for making shot and shells. Besides these general divisions, there are several other varieties of iron whose qualities depend upon the proportion of carbon, and the state in which it is found in the metal. The color and texture of cast iron depend greatly on the size of the casting and the rapidity of cooling. See [Ordnance, Strains upon].
Wrought iron was among the earliest metals employed in the construction of cannon, but in consequence of the defects which almost invariably accompany the forging of large masses, it was superseded by bronze and cast iron to a great extent. Wrought iron is softer than cast iron, and, being pure iron, is more liable to be corroded by the action of the atmosphere and products of combustion of the powder; it possesses also considerable ductility. The tensile strength of wrought iron, which under the most favorable circumstances is double that of the best cast iron, depends on the character of the crystalline structure, and the manner of applying the tensile force, or in other words, wrought iron offers the greatest resistance to a force of extension when the structure is fibrous, and the force acts in the direction of the fibres. The practical difficulties of rapidly cooling large masses so as to form small crystals, and compressing them by hammering, rolling, or otherwise to develop and give a particular direction to the fibre, have not thus far been wholly surmounted. On the contrary, large masses are generally found to contain such internal defects as false welds, cracks, and a spongy and irregularly crystalline structure, arising from the more rapid cooling of the exterior surface.