A single shot 37mm gun with a high muzzle velocity, namely 1000 metres per second, the F-21 is the primary weapon for defense against torpedo boats and aircraft, although the single shot, as compared to the 'magazine' of their contemporary Alqatat Gunsmiths 40mm Gun Mark II, reduced the rate of fire to a mere thirty rounds per minute. The 700 grams heavy fixed and cased charge had an effective ceiling of 2,000 metres and a maximum range of 8,500 metres at 37.5 degrees. They were used on virtually every vessel of the Lutetiian Navy.

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