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The Vickers Class "S" was a 40 mm gun used to arm British aircraft for attacking ground targets in the Second World War.

The Vickers Class "S" 40 mm gun was developed in the late 1930s as an aircraft weapon. The ammunition was based on the 40x158R cartridge case of the naval 2 pdr Anit-aircraft gun (the "Pom-pom"). The weapon was a long-recoil design derived from the 37 mm 1½pdr C.O.W. gun.The gun was originally intended as a bomber defensive weapon and was tested as such in a turret fitted to a modified Vickers Wellington II. This was not adopted for service, but when the need to attack tanks from the air was identified the "S" gun was chosen and special armour-piercing ammunition developed.

Two underwing guns were fitted to Hawker Hurricane IID fighters which were issued to No. 6 Squadron RAF. They served in North Africa from mid-1942 where they achieved considerable success; claims included 148 tanks hit, of which 47 were destroyed, plus nearly 200 other vehicles. However, they suffered heavy losses, mainly to ground fire (the Hurricanes were poorly protected) and also lacked effectiveness against the Tiger tank. In 1944, the aircraft served in the Far East, mainly firing HE ammunition against road and river transport.

Tests in the Far East showed a high level of accuracy, with an average of 25% of shots fired at tanks striking the target. Attacks with HE were twice as accurate as with AP, possibly because the ballistics were a closer match with the .303 inch Brownings used for sighting (the HE shell was lighter and was fired at a higher velocity). By comparison, the practice strike rate of the 60 pdr RPs (rocket projectiles) fired by fighter-bombers was only 5% against tank-sized targets. Operational Research following the Normandy battles of 1944 revealed that in action this fell to only 0.5%, presumably because of problems in making the complex mental calculations about the trajectory of the slow-accelerating rockets, although the effect of a salvo of RPs on the morale of tank crews was admittedly considerable.

2-pounder gun Mark XIV : a rival development to the Vickers S