British standard ordnance weights and measurements

The British standard ordnance weights and measurements for the artillery were established by the Master General of Ordnance in 1764, and these were not altered until 1919, when the metric system was additionally introduced. This system has largely been replaced by defining the weapon in terms of the measurement of the calibre, which is the standard today for most weapon systems in use by the world's armed forces.

Source: Wikipedia — British standard ordnance weights and measurements (CC BY-SA 4.0)