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9mm refers to the caliber of the bullet. Caliber is defined as the rough diameter of the projectile expel from the barrel. Some bullets do not coform to the exact diameter so the number is rounded. For example, a British 303, a Winchester 30.06, a 30-30, and a .300 Win Mag are all "30 caliber" but measure different diameters (allbeit very minor differences. A 9mm firearms, by the way, may utilize different cartridges. A "9mm Luger" measures 9mm by 19mm long where as the Soviet Union designed their firearms with a 9mm by 18mm cartridge which is not compatible--go figure.

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16y ago

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