Like Mk II, it fired existing 2pdr ammunition, large quantities of which remained after the First World War. That limited it to a muzzle velocity of 1900ft/sec, which by the late 1930s was clearly insufficient. The first operational mountings were ordered under the 1930–1 Estimates: twelve Mk M octuple mountings and six directors.58 The initial eight-barrel version was designated Mk M and later Mk V.59 During the 1930s a modified Mk VI mounting was ordered; it was standard during the Second World War.
By 1927 it had been decided that rounds should all be HE with sensitive-enough fuses to burst when hitting the fabric of an aircraft. Since the shell would not make a visible burst unless it hit, a proportion of ammunition had to be tracers. For initial trials the prototype mounting was connected to an Adventure-type director. Based on earlier trials in the cruiser Dragon, it was thought that a single control officer could lay and fire the mounting and also spot, but that proved to be too much for one man. A target moving almost directly at the mounting was relatively easy to hit, but it was much more difficult to hit a target with a high crossing speed; at any great range tracers gave a misleading impression. Spotting would have been easier if shells had been time-fused, bursting at the expected range. However, that would have required an automatic fuse-setter, an unacceptable complication.
Within a few years the Royal Navy considered several alternative ways to aim light anti-aircraft guns. Eyeshooting, which it eventually much favoured, had the gunner visualise the future position of the target and point at it using either a telescope or a forward area sight (an open sight). He might base aim on an estimate or he might use a telescope supported by some form of rate estimation. Any such method had to allow the gunner to override and select an alternative point of aim if the aircraft manoeuvred. Alternatively, course or speed sights could be set by a control operator for inclination, dive, and speed during an attack.
An alternative was hosepiping. No sights were used, the gunner relying on tracers to mark the trajectory of his rounds. In effect he was moving a hose of tracers. He relied on this visual aid to bring the trajectory onto the target and to keep it there. Lewis gunners used hosepiping because at very close range (500 yds and less) it was as efficient as eye-shooting and required less skill. To support it, the usual proportion of tracers was one to four rounds of ball in a Lewis gun.
The key argument favouring high muzzle velocity was that the target was not brought under fire until the time of flight of the initial rounds elapsed. If it jinked, it could not be brought under fire until another time of flight interval elapsed. In 1936 the DNO wrote that ‘from the control point of view reduction of time of flight is more important than any other single factor’. Time grew shorter and shorter as aircraft performance improved sharply through the 1930s.60
By 1928 Elliott Bros. was working on a prototype director. It was a simple sight on which allowance could be made for own speed, for the speed and course of the target, and for tangent elevation (range). The sight was laid and trained by one man, and data would automatically be sent to the mounting. It was understood that the weapon was intended mainly to defeat torpedo bombers. They placed themselves in a vulnerable position by flying low and straight for at least 15 to 20 seconds before releasing their torpedoes. That raised a problem. If the gun opened at excessive range it was unlikely to hit, but it might easily exhaust too much of the ammunition in a belt. Yet tracers had to be expended in order to tell that aim was correct for line, before the bomber settled down into its final run. DNO recommended firing a short burst (to establish line) at 2000 yds, then holding fire until about 1500 yds. By 1933 experience showed that to achieve 70 per cent hits it was necessary to know the range within 200 yds, inclination within 10–20° and speed within 20kts.
By this time there was a Mk I pom-pom director (initially described as a Director Sight). Unlike the director of an HACS, it was not connected to any sort of below-decks computer. It was a simple remote open sight which could transmit elevation and train orders to the Mk V pom-pom mounting. The back sight was set for enemy speed, dive or climb, and inclination. The developers recognised that such settings could not be correct for all ranges because the ratio of time of flight to range would vary with range. To deal with that problem, the foresight could be moved back and forth for range using a cam groove rather than a straight line. By 1934 directors were being modified so that enemy speed could be set more quickly, and also so that it could be released and reset to zero to deal with dive bombers – the emerging threat. The mounting was being altered to give a larger vertical pattern of shots at the expense of the lateral pattern – it was easier to aim for line than for range. Mk I* embodied detail improvements, but it too was basically a dummy sight. In 1935 DNO pointed out that Mk I was not suitable to engage dive bombers. Until the better Mk II was available, and new gun sights had been fitted, ships would have to rely on local control to fend off this kind of attack. Looking back in 1937, DNO wrote that Mk I had been of little use; control officers had to rely on eyeshooting.
A quadruple pom-pom on board the cruiser Bellona after the war. (State Library of Victoria)
A quadruple pom-pom from the 1945 Gunnery Pocket Book. Note that it has been modified for one-man operation, using a joystick. A major wartime lesson was that the usual two-man control (layer and trainer [pointer and trainer in US parlance]) was ill-adapted to fast-manoeuvring targets. That applied to directors as much as to gun mountings. (Photograph by Richard S Pekelney, Historic Naval Ships Association, courtesy of Mr Pekelney)
Mk I lacked a rangefinder, but by 1933 (as above) range was clearly a vital input (by 1937 arrangements were in hand to provide all existing pom-pom directors with 1m rangefinders). In 1934 Mk I** production was stopped in favour of a new Mk II with a 4ft rangefinder and a change of range in time of flight mechanism (range integrator). Just as importantly, Mk II incorporated a forward area sight, the Control Officer’s Forward Area Sight (COFAS). While layer and trainer kept their telescopes on the target, the control officer could aim off using this sight to provide deflection due to enemy movement across the line of sight. COFAS made eyeshooting far more effective, because the control officer was pointing his sight in the direction he envisaged. Wind correction was entered by a sight-setter. That left the control officer free to concentrate on target speed and course. As delivered, Mk II offered synchronous (M-type) rather than step-by-step data transmission to the gun mounting. The ultimate requirement was full power control (RPC). As of 1936 a power-control gear for multiple pom-poms was being tested. Overall director design was kept as simple as possible, first deliveries being expected before April 1936. Given the gap between Mk I and Mk II production, some ships would be delivered without directors in 1935–6. In fact Mk II proved difficult to make, and production was unsatisfactory. Mk II* had Magslip transmission, and Mk IIA had an improved COFAS.
Mk III was a Mk II modified for high- rather than low-velocity ammunition (Mk III* had Magslip transmission). According to the May 1939 edition of Progress in Naval Gunnery, Mk II had not come up to expectation. Production of it and Mk III would continue only until capacity could usefully be switched to the fully tachymetric Mk IV. During the war Mk IIIs were rebuilt with tachymetric inputs as Mk IIIT.
The companion to the pom-pom was the quadruple 0.5in machine gun, seen here before the war aboard Exeter during a visit to the United States. Like its predecessor the single pom-pom and its successor the Oerlikon, this gun was controlled entirely by its gunner; it had no director. It is not clear to what extent this gun was exported just before the outbreak of war. It apparently armed the Argentine Santa Cruz class destroyers built in the UK. It was to have armed the Brazilian destroyers which became the British Havant class as well as the new Turkish destroyers. It may have armed at least two of the rebuilt Greek destroyers. A British official