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The Mosquito FB Mk. XVIII (sometimes known as the “Tsetse“) of which one was converted from a FB Mk. VI to serve as prototype and 17 were purpose-built was armed with a Molins “6-pounder Class M“ cannon. This was a modified QF 6-pounder (57 mm) anti-tank gun fitted with an auto-loader to allow both semi- or fully automatic fire. 25 rounds were carried, with the entire installation weighing 1,580 lb (720 kg). In addition, 900 lb (410 kg) of armour was added within the engine cowlings, around the nose and under the cockpit floor to protect the engines and crew from heavily armed U-boats, the intended primary target of the . Two or four .303 Browning machine guns were retained in the nose and were used to “sight“ the main weapon onto the target. The Air Ministry initially suspected that this variant would not work, but tests proved otherwise. Although the gun provided the Mosquito with yet more anti-shipping firepower for use against U-boats, it required a steady approach run to aim and fire the gun, making its wooden construction an even greater liability, in the face of intense anti-aircraft fire. The gun had a muzzle velocity of 2,950 ft/s (900 m/s) and an excellent range of some 1,800–1,500 yd (1,600–1,400 m). It was sensitive to sidewards movement; an attack required a dive from 5,000 ft (1,500 m) at a 30° angle with the turn and bank indicator on centre. A move during the dive could jam the gun. The prototype HJ732 was converted from a and was first flown on 8 June 1943.
