15mm

The Besa machine gun was a British version of the Czechoslovak ZB-53 air-cooled, belt-fed machine gun (called the TK vz. 37 in the Czechoslovak army ).
The name came from the Birmingham Small Arms Company (BSA), who signed an agreement with Československá zbrojovka to manufacture the gun in the UK. The War Office ordered the weapon in 1938 and production began in 1939, after modifications.
It was used extensively by the armed forces of United Kingdom during the Second World War as a mounted machine gun for tanks and other armoured vehicles as a replacement for the heavier, water-cooled Vickers machine gun. Although it required a rather large opening in the tank's armour, it was reliable.

View More On Wikipedia.org