Prior to the attack on Pearl Harbor, the only heavy machine gun available for anti-aircraft defense was the free-swinging Browning water-cooled .50 caliber machine gun, which was lacking in range and firepower. To replace this weapon, there was need for a gun with more firepower but light enough to be manually handled.
The outstanding candidate for this was the Swiss Oerlikon company's 20mm gun, which was in service with Great Britain and which had been reviewed for service with the US Navy before the war. Eventually, on the grounds of it being the best available weapon, the Bureau of Ordnance approved the production in November 1940.
Reception was enthusiastic as a more than welcome replacement for the .50 caliber. Production was slow to quickly replace all of the .50 cal, so in the early months most air-defenses continued to rely on the older Brownings.
The 20 millimeter guns were the smallest anti-aircraft guns carried aboard the SLATER. As heavy machine guns, they were capable of firing 450 rounds per minute per barrel with an approximate range of 2000 yards. Each gun was manned by a crew of 4; a gunner, sight(range) setter, one loader per barrel. The gun was originally designed in Switzerland by the Oerlikon Company, hence the metric designation. Original mounts were on trunnions and required another crew member to raise and lower the trunnion. Later installations were the MK10 (tripod) mounts that eliminated this crewman.
Ammunition was held in sixty round drums that fitted on top of the breeches. After long periods of firing (about 240 rounds) the barrels had to be changed by the loaders, who wore asbestos gloves. The hot barrels were switched with spare barrels stored in cooling tanks welded alongside the guns. The ammunition drums were stored in lockers behind the gun mounts. When first placed in service in 1942, these guns were mounted singly and were aimed by open ring sights. After 1942, the lead computing stabilized MK14 relative rate gunsight was mounted on these guns for better accuracy and added the sightrange setter. In 1945, the twin mount became common which added an additional loader.
The career of the 20mm was quite short. It had extraordinary successes in 1942, 1943 and up to mid-1944, but with the advent of the kamikaze the 20mm was not adequate anymore. It was, however, retained because of the psychological effect that the ability to fire at the attacker had for the seamen, and twin and quadruple mounts had been tested and installed aboard several ships.
On V-J day, 1945, the SLATER carried nine of these twin mounts; three forward of the pilot house, four amidships on the 01 deck level (superstructure), and two on the fantail. Originally the SLATER had eight single mounts; four forward of the pilot house and four amidships on the 01 deck. These guns were not big enough to be very effective against the kamikaze. It was a common expression that when the twenties opened fire, the plane was in so close it was time to hit the deck.
Year of Construction: 1940