12. Jump.—Owing to the shock of discharge, a vibratory or wavy motion is set up in the barrel, which slightly affects the line of departure of the bullet as it leaves the muzzle, and which is known as “jump.” The causes and extent of jump are dealt with in the textbook of Small Arms. The effect of jump is allowed for in the sighting of the rifle, and will not be taken into account by the soldier in aiming.
13. Drift.—Drift is the term used to express the lateral deviation of the bullet after it has left the barrel. This is due to the direction of the rifling, which causes the bullet to rotate from right over to the left in its flight, so that the point works over slightly to the left, owing to gyroscopic action. The consequent increased air-pressure on the right side of the bullet forces it to the left as it flies. The deflection due to drift at distances within 1,000 yards is negligible, and need not be taken into account by the soldier in individual firing. Beyond 1,000 yards up to the limit of effective rifle fire at about 1,400 yards, drift will carry the bullet about 7 feet to the left. This deflection, if necessary, will be taken into account by fire-unit commanders in directing concentrated fire at narrow-fronted targets at longer ranges.
14. Effect of Fixing the Bayonet.—(i) When the bayonet is fixed to the muzzle of the rifle, its weight checks the jump, and in consequence slightly affects the position of the muzzle at the moment of the departure of the shot, and the primary direction given to the bullet.
(ii) Short Magazine Lee-Enfield Rifle.—With Mark VI ammunition, the accuracy of the short magazine Lee-Enfield rifle is not appreciably affected by fixing the bayonet. With Mark VII ammunition, the soldier will aim down slightly in firing with fixed bayonet at ranges up to 600 yards.
(iii) Lee-Metford or Charger-Loading Lee-Enfield Rifles.—With Mark VII ammunition, fixing bayonets has a negligible effect. When firing Mark VI ammunition from the Lee-Metford or Charger-loading Lee-Enfield rifles with fixed bayonets at ranges up to 600 yards, the soldier should aim up very slightly, taking care, however, in doing so to aim low in all cases, and to aim at the ground-line in firing at troops advancing towards him.
15. Effect of Resting the Rifle.—For practical purposes, the shooting of the rifle is not affected by resting the muzzle or any portion of the stock lightly on earth or other substance.
16. Effect of Oily Barrel.—The first round fired from an oily barrel is liable to follow an erratic course, the rifle throwing sometimes high, sometimes low, and at other times to the right or left. A dry rag should therefore be passed through the bore before practice is commenced.
Section 9.—Dangerous Space—Ricochets—Firing Up and Down Hill.
1. Dangerous space may broadly be defined as the whole ground covered by the trajectory of a bullet from the point where it could first come in contact with the top of an object fired at to the point where it falls to the earth (Figs. 4 and 5, and Figs. 6 and 8).
2. Extent of Dangerous Space.—The extent of dangerous space depends upon the following factors: