A bullet is a projectile propelled by a firearm, sling, slingshot, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration. The word "bullet" is sometimes colloquially used to refer to ammunition in general, or to a cartridge, which is a combination of the bullet, case/shell, powder, and primer. This use of 'bullet', when 'cartridge' is intended, leads to confusion when the components of a cartridge are discussed or intended. See the reference section for more detail.Bullets fired from slings, slingshots, and many airguns (including BB guns) travel well below the speed of sound (about 340 m/s or 1115 feet/s in dry air at sea level). Low-power handguns have muzzle velocities generally less than the speed of sound (subsonic), while bullets fired from high-power handguns (such as a .44 Magnum) and from rifles have an initial speed faster than the speed of sound, meaning they are supersonic and thus can travel a substantial distance and even hit a target before a nearby observer hears the "bang" of the shot. Bullet speed through air depends on a number of factors such as barometric pressure, humidity, air temperature, and wind speed.