A rifle is a firearm that uses a series of spiral grooves in its long barrel to spin the projectiles it fires, increasing their range and stability. Without rifling, a bullet can tumble as it flies, rendering it inaccurate and reducing the amount of kinetic energy it can transfer to the target. The first rifled barrels were developed in Germany at the end of the 15th century.
When a rifle is fired, the firing pin strikes the primer on the back of the bullet and triggers the inside gunpowder. As it burns, the gunpowder produces expanding gas, driving the bullet forward into the barrel. The rifling grooves channel the bullet and gas into a spiraling pattern, spinning the projectile as it moves down the barrel and then out the far end.
Modern rifles are organized into categories based on their reloading mechanism. Bolt-action rifles force the user to cycle the bolt manually between shots, each time ejecting any spent shell and cycling a new bullet into the chamber. Semi-automatic rifles use either an internal mechanism or siphoned-off gas from a fired bullet to drive the bolt after each shot, allowing the user to fire single shots quickly by pulling the trigger repeatedly. Fully automatic rifles will continue to fire and cycle as long as the trigger is held down by the shooter.