The Ordnance Quick-Firing 6-pounder 7 cwt, or just 6-pounder, was a British 57 mm gun, serving during the Second World War as a primary anti-tank gun of both the British and United States Army (as the 57 mm Gun M1). It was also used as the main armament for a number of armoured fighting vehicles. Although designed before the start of the war, it did not reach service until the North African Campaign in April 1942. There, it replaced the 2-pounder as an anti-tank gun, allowing the 25-pounder gun-howitzer to revert to its intended artillery role. Development and production Development Limitations of the existing 2-pounders were apparent even as the gun entered service and an effort was made to replace it with a much more capable weapon starting as early as 1938. The Woolwich Arsenal was entrusted with the development of a new gun with a calibre of 57 mm. Guns of this calibre had been employed by the Royal Navy from the late 19th century and manufacturing equipment was available. The gun design was complete by 1940 but the carriage design took until 1941. The production was further delayed by the defeat in the Battle of France. The loss of equipment – most of the heavy equipment of the British Expeditionary Force (BEF) was left behind in France during Operation Dynamo – and the prospect of a German invasion made re-equipping the army with anti-tank weapons an urgent task, so a decision was made to carry on the production of the 2-pounder, avoiding the period of adaptation to production and also of re-training and acclimatisation with the new weapon. It was estimated that 100 6-pounders would displace the production of 600 2-pounders. This had the effect of delaying production of the 6-pounder until November 1941 and its entry into service until May 1942. Unlike the 2-pounder, the new gun was mounted on a conventional two-wheeled split trail carriage. The first mass production variant – the Mk II – differed from the pre-production Mk I in having a shorter L/43 barrel, because of the shortage of suitable lathes. The Mk IV was fitted with an L/50 barrel, with muzzle brake. Optional side shields were issued to give the crew better protection but were apparently rarely used. The 6-pounder was used where possible to replace the 2-pounder in British tanks, requiring work on the turrets, pending the introduction of new tanks