Grooves

Spiral grooves or “rifles” cut into the surface of the bore of fire-arms, have the effect of communicating a rotary motion to a projectile around an axis coincident with its flight. This motion increases the range of the projectile, and also corrects one of the causes of deviation by distributing it uniformly around the line of flight. There are two kinds of twist used,—uniform and increasing. Increasing twists are supposed to give greater accuracy, and are well adapted to expanding projectiles which have but short bearing in the grooves. They are also used for stud projectiles. The uniform twist is generally applicable and has many advantages. In small-arms at the present time the bullet used has such a long bearing in the grooves that an increasing twist is not desirable. The ordinary twist is right-handed, giving a drift to the right. The gras , the official arm of France, has a left-handed twist. In regard to number and form of grooves authorities differ. The poly groove (or many small grooves) system has given very accurate results, especially in breech-loading cannon. It is in great favor on the continent of Europe. The French system, also used in a modified form in Woolwich and Elswick guns, consists of a few deep grooves in which the studs of the projectile move. The objection to this system is the weakening of the gun and the great strain when fired. For expanding projectiles, experiment shows that broad and shallow grooves with a moderate twist give range, endurance, accuracy of fire, and facility in loading and cleaning the bores. The proper twist to be given to the grooves depends on the length, diameter, and initial velocity of the projectile used; the most suitable twist is best determined by experiment. Other things being equal, the longer the projectile the more rapid the rotation nece ary to steady it; the greater the diameter the le rapid the rotation. As the initial velocity is increased, the rotation must be increased. The farther forward the centre of inertia of the projectile, the le the tendency to tumble. The tendency in modern small-arms is to shorten the twist on account of large charges and long bullets. Grooved bullets are best suited to broad shallow grooves; patched bullets to a greater number of sharp-edged grooves.