A bullet is a kinetic projectile, a component of firearm ammunition that is shot from a gun barrel. The term is from Middle French, originating as the diminutive of the word boulle (boullet), which means 'small ball'. Bullets are made of a variety of materials, such as copper, lead, steel, polymer, rubber and even wax. Bullets are made in various shapes and constructions (depending on the intended applications), including specialized functions such as hunting, target shooting, training and combat. Bullets are often tapered, making them more aerodynamic. Bullet sizes are expressed by their weights and diameters (referred to as 'calibers') in both imperial and metric measurement systems. For example: 55 grain. 223 caliber bullets are of the same weight and caliber as 3. 56 gram 5. 56mm caliber bullets. Bullets do not normally contain explosives (see Incendiary ammunition and Exploding bullet), but strike or damage the intended target by transferring kinetic energy upon impact and penetration (see terminal ballistics). Bullets are available singly (as in muzzle-loading and cap and ball firearms), but are more often packaged with propellant as cartridges ('rounds' of ammunition). Bullets are components of paper cartridges, or (much more commonly) in the form of metallic cartridges.