=3. Technical Terms.=--The different technical terms used in this chapter in explaining the theory and application of rifle fire will be found among the Definitions (p. xxii). Some of these terms are illustrated in Figs. 2 and 3. In addition, the following technical terms may conveniently be quoted in this paragraph: (i) =Individual Fire.=--Individual fire is fire opened by individual soldiers without orders from a fire-unit commander. When men fire individually, each one selects his target, estimates the range, and regulates his fire according to his own judgment. (ii) =Collective Fire.=--Collective fire is the fire of a number of men combined for a definite purpose under the orders of a fire-unit commander, who indicates targets, gives ranges, and controls fire as to rate, etc. _Note._--Collective fire is necessarily controlled fire. _Individual fire will also be controlled if circumstances permit._ =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.