What is 12 bore in mm

12 bore (12 gauge shotgun) has a nominal bore diameter of approximately 18.5 mm (0.729 inches). The bore designation system originated from the number of lead balls of bore diameter required to make one pound: a 12-gauge bore could fit twelve lead spheres of bore diameter to equal one pound weight. This inverse system means smaller gauge numbers indicate larger bores: 10 gauge (19.7 mm) is larger than 12 gauge (18.5 mm), which is larger than 20 gauge (15.6 mm).

The 12 gauge is the most popular shotgun bore worldwide for hunting, sport shooting, and law enforcement applications, offering versatile performance across different ammunition loads from light target shot to heavy slugs. The actual bore diameter can vary slightly between manufacturers (typically 18.4-18.7 mm) as chamber specifications, choke constrictions, and forcing cone dimensions differ. When measuring shotgun bores or selecting barrel components, the nominal 18.5 mm provides adequate reference for general purposes, though precise gunsmithing work requires actual measurement of specific barrels. The gauge system continues in shotgun terminology despite being non-intuitive, as switching to direct millimeter designation would require massive industry changes to ammunition standards, chamber specifications, and established nomenclature recognized by shooters globally. The exception is the .410 bore shotgun, which despite the nomenclature suggesting caliber measurement, is actually a bore designation roughly equivalent to 67 gauge—though universally referred to by its approximate bore diameter rather than gauge number.