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The "Big Gun" Mosquito


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Helping this old man out, our hero @Bremspropeller found the reference I could not recall off the top of my cranium. Of all the Mosquito related titles I have, Mosquito: The Original Multi-Role Combat Aircraft has the best discussion of the development of the FB.XVIII (or FB.Mk.XVII or Mk.XVIII if you prefer). Absent the photographs, here's what Graham Simons wrote.



The ‘Big Gun’ Mosquito

The Mosquito fighter, as explained earlier, had been planned in two forms: a home defence variant and a convoy escort type requiring greater endurance and long range. The convoy escort type was typified by the FB.Mk.VI, which first went into service with 333 (Norwegian) Sqn of Coastal Command. An important duty falling to Coastal Command was the destruction of enemy submarines entering or leaving the Bay of Biscay ports to attack Allied convoys. This was a growing menace, that required considerable firepower to combat it. Therefore, on 19 March 1943, J E Serby from the Ministry of Aircraft Production asked Ronald Bishop at De Havilland to investigate the possibility of installing a six-pounder cannon, weighing l,800 lb, in the Mosquito.


This request originated from discussions within the Air Ministry for a replacement of the 40mm anti-tank gun fitted with such success to the Hawker Hurricane IID used in North Africa. Under the leadership of GF Wallace, a series of ground firing tests was carried out by the RAF’s Gun Section. The gun was found to be trouble-free, although feed unit problems were feared under the stress of actual combat. The head of the department sent a favourable report to the Controller of RAF Research and Development, Air Marshal Sir Ralph S. Sorley, who was instructed by the Air Staff to make the necessary requests to the aircraft manufacturers.


The gun intended was an adaptation of a weapon designed originally for naval use, with the addition of an automatic feed. It was designed and manufactured by Molins Machine Company, one of the world’s leading manufacturers of cigarette-making, packing and handling equipment. Desmond Molins had already done considerable work on the automatic feed, intending to adapt the gun to be fitted to a tank-busting armoured car.


The automatic feed mechanism was to store rounds in groups of four or five, with an electrical drive to move the next group into position over the breech feed. The upward angled magazine took up relatively little space, and allowed the heavy shells to be fed automatically into the weapon without the use of case links. While working on the gun’s feed unit, Molins discovered that the gun needed modification to allow the recoil mechanism to operate the magazine. In time this gun became known as the ‘Molins Gun’ or, in official parlance, the ‘Airborne Six-Pounder Class M Gun’.


Bishop, while knowing little of the Molins gun, stated that rough calculations indicated that few problems would be encountered in dealing with the envisaged 8,000lb recoil reaction. De Havilland had already studied the mounting of a 94mm/3.7 inch anti-aircraft gun in the Mosquito and, during December 1942, had already made detailed weight estimates for a ground-attack version with extra armour plating.


Serby therefore gave instructions in April 1943 to proceed with the prototype installation. De Havilland had previously adopted policies to ensure that every modification of the Mosquito family was developed as rapidly as possible. Illustrating the excellence of their system, just one day later they had cut the nose from a crashed machine, installed a normal six pounder gun measuring over twelve feet in length, and fired the weapon into the Hatfield butts to study the blast effects on the nose. During the first week of May another mock-up was made to see how the ammunition would be placed and, later the same week, a FB.Mk.VI, HJ732, was taken into the experimental shop for the first ‘big gun’ to be installed. This task was completed on 6 June, allowing the test firing of all 22 rounds in the magazine in one burst into the sand of the stop-butts; when fired into a jump-card set at 400 yards range, the mean point of impact was ten inches.


This adaptation of the Mosquito design was allocated the designation Mk.XVIII and the code-name of ‘Tsetse’ after the African tsetse fly, an insect that causes human sleeping sickness and animal trypanosomiasis. To fit correctly, the gun had to be mounted four inches to starboard of the aircraft’s centre-line, and aligned to fire at a slightly downward angle of 3 ¾ degrees to the aircraft’s horizontal datum. It was not overlooked that when fired, the gun emitted a flame between 15 and 30 feet long and therefore required a flash-eliminator.


The aircraft made its first flight two days later. No difficulties were encountered with either recoil or blast, so the machine was passed to A&AEE Boscombe Down for air firing trials. In the first week alone 100 rounds were fired into the butts to try to cure the problem of overpowering the feed unit drive during combat, a problem that had been predicted by the RAF’s Gun Section. These stoppages occurred because rounds failed to reach the correct position when the feed mechanism was subjected to loads over 2.5G on the final run. On 22 June the aircraft was returned to Hatfield for further modifications.


Another problem arose during the testing of the gun. It was known that when any weapon was mounted in the closed compartment of an aircraft, and most especially if the barrel was facing forward directly into the airflow, some of the gases produced from firing were blown back when the breech was opened to eject the spent shell case. However, the ‘blow through’ created by the six-pounder when the breech was open was incredibly powerful; it could be likened to a 57mm-diameter column of air moving at up to 350 mph into the fuselage of the Mosquito. It was discovered that the intense air pressure was enough to hinder the operation of the automatic loading mechanism, not to mention the effects of gas fumes on the crew in the cabin. To overcome this problem - which existed on all guns but was particularly noticeable on the Class M gun because of the barrel diameter - Desmond Molins set about devising a solution. The result was a blast tube that contained a multi-sectional, spring-steel muzzle cap that screwed on to the end of the barrel that was provisionally patented under No.581817 on 12 January 1943. This date is interesting, for it demonstrates that research into fitting large bore guns into the fuselages of aircraft must have been going on for some time before the Ministry of Aircraft Production notified De Havilland of its thoughts on 19 March 1943.


The muzzle cap was connected to the automatic loading mechanism, and only closed when the breech was actually open. It was also designed failsafe, in that if the mechanism did jam in the closed position, it was possible for the shell to be fired through the spring steel strips.


HJ732 returned to Boscombe Down for further trials, during which the valuable prototype was nearly lost to the Germans when navigational errors were made one weekend. Wing Commander Garland, accompanied by David King of De Havilland, was conducting tests over Lyme Bay in the West Country when the pilot contacted Boscombe Down for a fix and course to steer after the air firing trials had been completed. He was given it, and set the course on his compass. As they flew out to sea they saw haze on the horizon, which turned out to be the French coast! It seems that the corporal on radio duty had given them not a course to steer, but the reciprocal. Luckily the Wing Commander made a successful, if later than planned, landing at Boscombe Down.


Meanwhile a second machine had been fitted with extra armour around the nose and cockpit, weighing 900 lb in all. The first accuracy shoot took place two weeks later against a tank target at Larkhill on Salisbury Plain. The 57mm cannon could be fired either single shot or, with the automatic feed engaged, one shell could be fired every one and half seconds. Many people observing the tests wondered how the Mosquito structure could stand up to such stresses and still stay together the answer of course lay in the fact that the wooden structure was flexible and therefore absorbed shocks safely and efficiently.


Following this successful testing, work began at Hatfield to convert 30 Mosquito FB.Mk.VIs into Mk.XVIIIs. Crates containing Molins Guns were received and placed in secure storage until needed.


The firing trials continued, however, and in July and August HJ732 was back at Boscombe Down to help devise and practise operational tactics, using a dummy submarine conning tower built of wood and rigged up on the ranges. It was found that if a straight and level run-in at 50 to 100 feet altitude was made, it was possible for the pilot to fire off around seven shots during the mile-long approach. The gun was exceptionally accurate, with an average of six out of the seven shots finding the target. The rifling of the barrel began to show signs of wear and tear after 300-500 rounds had been fired, but this was considered acceptable if it translated, as expected, into 20 sorties each expending a full load of ammunition. A more worrying sign was the damage which 450 rounds - regarded as ‘excessive use’ - inflicted on the undersurface of the inner flaps of the aircraft, notably to the starboard side. This ‘sucking away’ effect was cured by fitting a heavier skin and by adding strengthened longitudinal ribs to the existing flap structure.


Blast effect problems also affected the nose area, with cracks appearing between the ‘Big Gun’ and the four Browning 0.303in machine guns. It was decided to remove the outer pair of machine-guns and strengthen the area with tie-rods fitted between the gun mounts and nose cone. A drop in fire-power was averted by enlarging the ammunition tanks and thus doubling the firing time of the remaining machine-guns.


By October three production aircraft had reached Boscombe Down. A minor modification, the fitment of 65 gallon fuel tanks in the fuselage, allowed the aircraft to range out far over the Bay of Biscay. A big-gunned, long-range Mosquito offered plenty of promise.

With many of the technical problems overcome, operating procedures could now be worked out. It was thought that the Barr and Stroud Mk.IIIa reflector sight, normally mounted in turrets and on free guns, could be used since it gave a better peripheral view and, when fitted with a dimming screen to reduce the reflections from the surface of the sea, it would be better than the larger GM2 fighter gunsight. The alignment of the Brownings and the six-pounder was not the same and therefore two graticules, or aiming dots, were incorporated in the sight with the central one for the Molins Gun and a slightly higher one for the machine-guns.


Simons, Graham. Mosquito: The Original Multi-Role Combat Aircraft . Pen and Sword. Kindle Edition.

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thx busdriver

cant wait to hear the bloody thing fire, apparently deafening in the cockpit, flame enveloping the plane (and assume that is with the flash illiminator fitted) and with a "robust" jolt and seems even the reload noise was akin to close miss by flak showering plane with shrapnel (know youve read that book as well).

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This is a decent little documentary about the Tse Tse Mosquito by the De Havilland Aircraft Museum. It mostly consists of interview with a navigator and a pilot of these aircraft.



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