A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves ("rifling") cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile (for small arms usage, called a bullet), imparting spin around an axis corresponding to the orientation of the weapon. When the projectile leaves the barrel, this spin lends gyroscopic stability to the projectile and prevents tumbling, in the same way that a properly thrown American football or rugby ball behaves. This allows the use of aerodynamically-efficient pointed bullets (as opposed to the spherical balls used in muskets) and thus improves range. The word "rifle" originally referred to the grooving, and a rifle was called a "rifled gun." Rifles are used in warfare, hunting and shooting sports.
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 military parlance, the word "gun" refers to 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 rifles are capable of firing more than one round per trigger squeeze; some fire in a fully automatic mode and others are limited to fixed bursts of two, three, or more rounds per squeeze. Thus, modern automatic rifles overlap to an extent in design and function with machine guns. In fact, many light machine guns (such as the Russian RPK) are adaptations of existing automatic 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. Modern military rifles are fed by box magazines, while machine guns are generally (but not always) belt-fed. While machine guns are often crewed by more than one soldier, the rifle is an individual weapon.
The term "rifle" is sometimes used to describe rifled weapons firing explosive shells; for example the recoilless rifle.