Typically, a bullet is propelled by the contained deflagration of an explosive compound (originally black powder, later cordite, and now nitrocellulose), although other means such as compressed air are used in air rifles, which are popular for vermin control, hunting small game, formal target shooting and casual shooting ("plinking").
In most armed forces the term "gun" is incorrect when referring to small arms; in the military, the word "gun" means an artillery piece or crew-served machine gun. Furthermore, in many works of fiction a rifle refers to any weapon that has a stock and is shouldered before firing, even if the weapon is not rifled or does not fire solid projectiles (e.g. a "laser rifle").
Formerly, rifles only fired a single projectile with each squeeze of the trigger. Modern assault rifles are capable of firing more than one round per trigger squeeze; some fire in a fully automatic mode and others are limited to bursts of three to five rounds per squeeze. Thus, modern assault rifles overlap somewhat with machine guns. In fact many light machine guns (such as the Russian RPK) are adaptations of existing assault rifle designs. Generally, the difference between an automatic rifle and a machine gun comes down to weight and feed system; rifles, with their relatively light components (which overheat quickly) and small magazines, are incapable of sustained automatic fire in the way that machine guns are. While machine guns may require more than one operator, the rifle is an individual weapon.
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