Bore (Gauge)
Modern equivalent
12-bore = 18.5 mm; 20-bore = 15.6 mm
The diameter of a gun barrel, expressed as the number of spherical lead balls of that diameter which together weigh one pound.
The bore is that rare thing, a unit of length defined entirely by weight, and it runs backwards: the larger the number, the smaller the barrel, because more small balls are needed to make up the pound. A 12-bore takes twelve balls to the pound and measures 18.5 millimetres; a 20-bore takes twenty and measures only 15.6. The system was standardised by the Birmingham and London Proof Houses, which have tested and stamped British firearms since the seventeenth century, and it remains in daily use across the shooting trade.