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 This system has largely been replaced by defining the weapon in terms of the measurement of the calibre, which is the standard today for most weapon systems in use by the world's armed forces. The 18th century standards were based on a projectile's weight, which dates back to use of muzzle loaded cannons that fired solid cannonballs. The bore designations are only an approximate relationship to the actual weight of the projectile when it was applied to modern artillery. The table below lists the metric and Imperial calibres of various British weapons, which used the standard after 1919:
+NameTypeCalibreMetricImperialOrdnance QF 1-pounder "pom pom"Infantry gun/AA gun37 mm1.457 inchOrdnance QF 2-pounderAnti-tank gun40 mm1.575 inchOrdnance QF 2-pounder "pom pom"Anti-aircraft gun40 mm1.575 inchOrdnance QF 3-pounder VickersNaval gun47 mm1.85 inchOrdnance QF 6-pounderAnti-tank gun57 mm2.244 inchOrdnance BL 10-pounder Mountain gunMountain gun69.8 mm2.75 inch12-pounder (multiple types)Light field gun76.2 mm3 inchOrdnance QF 13-pounderLight field gun76.2 mm3 inch15- pounder (several types)Field gun76.2 mm3 inchOrdnance QF 17- pounderAnti-tank gun76.2 mm3 inchOrdnance QF 18- pounderField gun83.8 mm3.3 inchOrdnance QF 20-pounderTank gun83.8 mm3.3 inchOrdnance QF 25-pounderGun-howitzer87.6 mm3.45 inchOrdnance QF 32-pounderTank gun94 mm3.7 inchOrdnance QF 60-pounderHeavy field gun127 mm5 inch
Terminology
When used with British standard nomenclature: BL is short for "Breech Loading", but generally means not QF, i.e. separate cordite bags rather than a cartridge case. ML is short for "Muzzle Loading". PR is short for "Pounder", e.g. 20 Pounder can be shortened to "20-PR". pdr is also a common shortening of "pounder", e.g. 17pdr. QF is short for "Quick Firing", indicating the weapon is breech-loaded with the propellant in a cartridge case which also made the breech seal, allowing faster loading and firing.
See also Pounds as a measure of cannon bore Glossary of British ordnance terms
References Rottman, Gordon L.: Elite 124 - World War II Infantry Anti-Tank Tactics, Osprey publishing, p. 16 Collins, A.R. "British Cannonball Sizes" https://www.arc.id.au/Cannonballs.html