A bullet is a rock-hard projectile propelled by a firearm or air gun and is normally made from metal (usually lead). A bullet (in contrast to a shell) does not contain explosives, and damages the intended target solely by imparting kinetic energy upon impact. Modern bullets for firearms are generally part of a cartridge, also known as a round. In contrast, bullets for air guns are not part of a cartridge. The word "bullet" is sometimes used to refer to the grouping of bullet, case, gunpowder and primer more properly known as a cartridge or round.; the Oxford English Dictionary definition of a bullet is "a projectile of lead ... for firing from a rifle, revolver etc."