In guns, particularly firearms, caliber or calibre is the approximate internal diameter of the barrel, or the diameter of the projectile it fires, in hundredths or sometimes thousandths of an inch. For example, a 45 caliber firearm has a barrel diameter of .45 of an inch. Barrel diameters can also be expressed using metric dimensions, as in "9mm pistol." When the barrel diameter is given in inches, the abbreviation "cal" (for "caliber") can be used. For example, a small-bore rifle with a diameter of 0.22 inches can be referred to as .22 or a .22 cal; however, the decimal point is generally dropped when spoken, making it a "twenty-two caliber" or a "two-two caliber rifle".