Field artillery materiel

by Unknown

(a) An inner steel tube which forms a support on which the wire is wrapped and in which the rifling grooves are cut.

(b) Layers of wire wrapped upon the tube to increase its resistance by the application of an exterior pressure as well as to add to the strength of the structure by their own resistance to extension under fire.

(c) One or more layers consisting of a steel jacket and hoops placed over the wire with or without shrinkage. The jacket generally furnishes longitudinal strength to the guns, and the breech block is screwed into the jacket, or into a breech bushing, which is screwed into the jacket.

The principal advantages of this type of gun over the built-up is economy of manufacture and greater facilities for inspection of materiel in the layers over the tube. The wire wrapping has itself a large reserve of strength due to the high elastic limits that may be given it. Two methods are used to wrap the wire: (a) at constant tension (b) at varying tension so that when the gun is fired with the prescribed pressure, all layers of wire shall be subjected to the same tangential stress. The latter method is theoretically better, but because of the ease of manufacture, together with the large factor of safety possible, the wire is usually wrapped at a constant pressure.

THE BUILT-UP GUN.

All army guns except small howitzers or mortars are of the built-up or wire-wrapped type. Built up guns of less than 5” caliber, or howitzers up to 8” caliber consist of an inner tube and a jacket shrunk onto this tube. The jacket covers the breech end of the gun and extends forward to the center of gravity. Built-up guns of larger caliber have one more layer of hoops in addition to the jacket, one layer of hoops usually extending to the muzzle.

The bore of the tube forms the powder chamber, the seat for the projectile and the rifled bore. Rifling consists of a number of helical grooves cut in the surface of the bore. The soft metal of the rotating band of the projectile is forced into these grooves causing the projectile to take up a rotary motion as it passes through the bore. This is necessary in order to keep the projectile stable in its flight.

TWIST.

By twist of rifling is meant the inclination of one of the grooves to the element of the bore at any point. Rifling is of two kinds: (a) Uniform twist, or that in which the twist is constant throughout the bore, (b) Increasing twist or that in which the twist increases from the breech towards the muzzle.

The twist of rifling is usually expressed in the number of calibers length of bore in which it makes one complete turn. The twist actually required at the muzzle to maintain the stability of the projectile varies with the kind of projectile and the muzzle velocity. If a uniform twist be used, the driving force on the rotating band will be at a maximum when the pressure in the guns is at a maximum—or near the origin of rifling (seat of the projectile). The increasing twist serves to reduce the maximum driving force on the band thus lessening the danger of stripping the band. This is its principal advantage over the uniform twist, though it also reduces slightly the maximum pressure in the gun. The principal disadvantage of the increasing twist is the continued change in form of the grooves pressed in the rotating band, as the projectile passes through the bore. This results in increased friction and a higher value for the passive resistance than with a uniform twist. (Note: greater ranges obtained by cutting grooves in projectile, principal used on the long range gun by the Germans.) If the twist increases from zero at the breech uniformly to the muzzle, the rate of change in the tangent to the groove is constant. A twist in this form offers less resistance than the uniform twist to the initial rotation of the projectile. To still further diminish this resistance a twist that is at first less rapid than the uniformly increasing twist and later more rapid has been generally adopted for rifled guns.