The "King George V" design had four 0.5-inch quadruple machine gun mounts, but in 1939 these were replaced by two Mark VI pom-pom mounts. In 1940, to combat air attack, four Unrotated Projectile mountings were fitted, on "B" turret, two on "Y" turret, one replaced a pom-pom mount added in 1939 at the stern. The pom-poms mounted in the "King George V" were designed and produced by Vickers Armstrongs as a result of a post-First World War requirement for a multiple mounting which was effective against close-range bombers or torpedo planes. The first model, tested in 1927, was superior to anything developed in other countries at the time and in 1938 the Mark VI* had a muzzle velocity of 2,400 feet per second, a 1.594-inch bore and a barrel length of 40 calibres. They fired a shell at a rate of 96–98 rounds per minute for controlled fire and 115 rounds per minute for automatic fire. The range of the Mark VI* was , at a muzzle velocity of 2,300 feet per second. The Mark VI octuple mount weighed 16 tons. The Mark VII quadruple mount weighed 10.8 tons if power operated; it could be elevated to 80 degrees and depressed to 10 degrees at a rate of 25 degrees per second which was also the rate of train. The normal ammunition supply on board for the Mark VI was 1,800 rounds per barrel. "King George V" introduced the Mk IV Pom-pom director to the Royal Navy in 1940, becoming the first ship in the world to feature gyroscopic target tracking in tachymetric anti-aircraft directors.