The Browning 0.303in Mark II was the standard gun used in the gun turrets on most British bombers during the Second World War. It was essentially the 1930 Pattern belt-fed Colt-Browning machine gun with a few minor modifications for British use. The gun was selected after competitive tests in 1934. BSA and Vickers were both given a license to produce the gun, and by the outbreak of war production was running flat out.
The original gun needed two changes to make it entirely suitable for British use. Pre-war tests revealed that the gun needed some modifications for use with British ammunition, which was cordite-based. In wartime conditions it was discovered that the muzzle extension was prone to fouling – many more rounds were being fired by each gun in combat that in testing. The RAF gun section redesigned the muzzle extension, chrome plating it to make the surface smoother and adding cooling fins. This modified version was designated at the Mark II and remained the standard gun used in British bombers for most of the war.
The Browning .303in gun was retained for too long in British use. It could be found in the vast majority of British bombers. It was used in many Frazer-Nash turrets, such as the FN-5, which was used in both the Wellington and Lancaster bombers. In contrast, the Americans used .50in machine guns in their bombers, while fighter aircraft were increasingly using 20mm cannon.
The Browning Mark II did have some important advantages. It was reliable, accurate, and available in large numbers. The longer range of the heavier guns was more useful in daylight operations, and was of little benefit at night, at least until radar assistance was available for gunners. The heavier calibre guns also used heavier ammunition – the .303 bullet weighed 0.4 oz, the .05 more than three times that much at 1.4 oz. The use of the heavier gun forced a reduction in potential bomb load, or in the amount of ammunition carried. Only towards the end of 1944 did the RAF begin to use 0.5in calibre machine guns in their bombers, in expectation of a return to day operations.
Weight: 21 lb 14 oz
Muzzle velocity: 2,660 ft/sec
Rate of fire: 1,150 rpm
Maximum Range: 3,000 ft
Length: 3 ft 8.5 in