Buy Handguns Ammo for sale online. In guns, particularly firearms, caliber (or calibre in British English) is the specified nominal internal diameter of the gun barrel bore regardless of how or where the bore is measured and whether or not the finished bore matches that specification.[1] It is measured in inches or in millimeters.[2] In the United States it is expressed in hundredths of an inch; in Great Britain in thousandths; in Europe and elsewhere in millimeters. For example, a “45 caliber” firearm has a barrel diameter of roughly 0.45 inches (11 mm). Barrel diameters can also be expressed using metric dimensions. For example, a “9 mm pistol” has a barrel diameter of about 9 millimeters. Due to the fact that metric and US customary units do not convert evenly at this scale, metric conversions of caliber measured in decimal inches are typically approximations of the precise specifications in US customary units, and vice versa. Buy Handguns Ammo for sale online