4. Rifling.—A gun-barrel is said to be rifled when it has spiral grooves cut down the bore. Rifling a barrel enables an elongated bullet to be used instead of the round “ball” of former days. The advantage of this form of bullet is that it has great weight in proportion to the surface directly opposed to the air. It has therefore great power of overcoming the resistance of the air, and thus maintaining its velocity and penetrating force[5] over greater distances than would otherwise be possible. Moreover, when the charge is fired, the bullet is forced into and follows the spiral grooves up the barrel, the effect of which causes it to leave the muzzle rotating or spinning on its longer axis. This spin tends to keep its point foremost, and therefore to ensure accuracy of flight.
5. Forces Acting on the Bullet.—Three forces act on the bullet: (a) The explosion of the charge, (b) gravity, and (c) the resistance of the air. The explosion of the charge drives the bullet forward. Gravity—the natural attraction which draws all unsupported bodies towards the centre of the earth with ever-increasing velocity—acts on the bullet immediately it leaves the muzzle. The resistance of the air causes the velocity of the bullet to decrease rapidly during its flight.
6. Trajectory.—The combined effect of these forces causes the bullet to travel in a curved line called the trajectory, the curvature of which becomes more pronounced the longer the bullet is exposed to their action. Thus, a Mark VI bullet leaving the muzzle of a service rifle at the rate of about 2,060 feet per second falls about 4½ inches below the line of departure in the first 100 yards. This drop is increased to about 20 inches at 200 yards. With Mark VII ammunition, giving a muzzle velocity of 2,440 feet per second, the drops at the above distances are about 3 inches and 13 inches respectively. Imaginary not actual curves of trajectories are shown in Figs. 6 and 8.
7. Method of Explaining Trajectory.[6]—(i) In explaining the trajectory to recruits it is not sufficient merely to show or draw a diagram representing a trajectory distorted in respect of height and range. When possible the actual path of the bullet through the air at various short ranges—say 400 and 800 yards—should be shown by means of discs raised on poles at every 100 yards, or some similar device.
(ii) It must be explained to the soldier that the further an object has to travel and the longer it is suspended in the air, the higher it must be thrown to counteract the force of gravity, because the greater the distance it travels and the longer it remains in the air the longer will the object be affected by the force of gravity. Consequently the longer the range the higher will the curve of the trajectory take the bullet in its flight, and the steeper will be the angle at which it will fall to the earth.
(iii) On the other hand, the shorter the distance an object is thrown, and the swifter its flight through the air, the less will it be influenced by the force of gravity during its flight, and the lower or flatter will its trajectory be in consequence. Thus, with the backsight adjusted for the distances in question, a Mark VI bullet does not rise above the height of a man on foot at 500 yards range, or above the height of a mounted man at 600 yards range. With Mark VII ammunition, the bullet does not rise above these heights at 600 and 700 yards range respectively.
8. Firing at Close Range without Altering Sights.—It is therefore evident that effective fire can be maintained within close range without alteration of the backsight. Apart from the flatness of the trajectory at close range, and the consequent inclusion of distances within close range in dangerous space (Sec. 9), there will seldom be opportunities for altering sights at close range on service, and necessary allowances for elevation must be made by aiming up and down (Sec. 21).
9. Elevation.—In order to allow for the fall of the bullet, owing to the force of gravity, it is necessary to direct,the line of departure as much above the object to be hit, as the bullet will fall below it at any given distance if the axis of the barrel of the rifle is pointed at the mark. This raising of the barrel to allow for the curve of the trajectory is termed giving elevation. The target must of necessity be kept in view. The rifle is therefore provided with sights, which permit the firer to give the elevation required whilst keeping his eye fixed on the mark.
10. The Sighting of Rifles.—(i) In the sighting of rifles a mean graduation for each range has been adopted, and a high general standard of accuracy for all practical purposes is thus obtained. Each rifle is carefully tested before issue, but it must be understood that no two rifles behave in an exactly similar manner, and that even if compensation could be made for every error in the sighting of the rifle before issue, the wear of parts and the loosening or tightening of screws, etc., would bring about faults from time to time which would affect the shooting of each rifle differently.
11. Need of Knowledge of Each Weapon.—It is therefore necessary that every man should study the shooting of his own rifle, and make himself acquainted with any incorrectness of the graduations marked on the backsight, in order that he may be in a position to give his rifle the correct elevation for the estimated or ascertained range of the target. At longer ranges the backsight elevation may be regarded as the best possible guide to errors under all conditions, or any error may be ascertained by using a long-range sighting target.[7]