Mk II
The Mk II Projector was developed after a request from the Royal Navy for a version that could be fired using steam in place of compressed air, since the steam trawlers (both fishing and minesweeping) had the former in plentiful supply. The Stokes design of the Mk I needed to be abandoned for this, since the harsh weather experienced by the trawlers invariably rusted the valves of the pneumatics. When steam was used in such a system, the water would condense in the pipes and prevent firing of the weapon. To solve this, a firing trigger was added, in place of the Stokes design in which the round would be fired automatically.
This new version was fitted to a wide variety of ships, from destroyers to minesweepers and motor gun boats. To demonstrate the weapon's versatility, a trial was arranged in Aldershot, Hampshire before Prime Minister Winston Churchill. No Mills rounds were brought, as it was assumed that some form of ammunition would be provided by the British Army, who were overseeing the trials. As it turned out, this was overlooked and the trial was delayed until one officer thought to bring out the bottles of beer that had been brought to serve at lunch. The smooth bore of the Projector allowed even these irregular projectiles to be fired successfully, with all striking the target with an explosion of glass and foam. The Prime Minister commented on the weapon afterwards, describing it as "A very good idea, this weapon of yours. It will save our cordite".
Regardless of the successful trials, The Mk II was highly inaccurate when fired at distant moving targets. Only a dozen or so aircraft were confirmed to have been downed by the weapon in its first year of service. It succeeded in convincing many more aircraft that the target vessel was more heavily equipped than it actually was, and a large number of reports were made about Luftwaffe aircraft turning away from an attack after salvoes from the ship's Holman Projector were launched. Within the Admiralty, the perception was that the Projector was a useful stop-gap weapon in the early years of the war, when other more effective anti aircraft weaponry, such as the Oerlikon 20 mm cannon, were in short supply.
Read more about this topic: Holman Projector