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The Oerlikon 20 mm: The Right Tool for the Job

March 18, 2019 | By Daniel Garas, Naval History and Heritage Command
Before it entered World War II, the United States Navy realized aviation would play a significant role in the upcoming conflict. In order to deal with this emerging threat, its ships would need to be outfitted with capable anti-aircraft systems. Throughout the interwar period, the Navy's Bureau of Ordnance worked on the development of anti-aircraft guns and automatic weapons of various types designed to counter the threat of aerial assault.

One area in particular where the Navy knew it needed to innovate was in effective, close-range anti-aircraft defense. The .50-caliber Browning machine gun lacked the hitting power and range necessary to bring down the fast-moving, monoplanes of the day. The quad 1.1-inch gun (nicknamed the "Chicago Piano") proved to be equally disappointing and frequently jammed. The solution came from an unlikely source; Germany.

Developed in the final days of World War I, Germany sold the rights to a 20 mm caliber cannon to a Swiss firm that eventually became the Oerlikon 20 mm. The weapon proved popular, and before the start of World War II, numerous countries obtained the manufacturing rights. This gave the weapon the unusual distinction of being utilized by both Allied and Axis forces during the war.

Adopted by the U.S. Navy in November 1940, the Oerlikon 20 mm was operated by a crew of four: the gunner, loader, spotter, and gun captain. It provided effective short-range defense against aircraft that the .50-caliber Browning or the quad 1.1-inch could not. Its simple blow-back design (unusual for a high powered autocannon), meant that force of the round would cycle the breech against a powerful recoil spring that wrapped around the barrel. With a cyclic rate of 480 rounds-per-minute, and fed by a drum magazine of 60 rounds, the weapon would keep firing as long as the firing lever was held back or until something broke.

According to Robert Wallace, author of "From Dam Neck to Okinawa: A Memoir of Anti-Aircraft Training in World War II," The secret to the weapon's success was that it consisted of simple components and was easy to fire and maintain. "There were only five parts that could break," wrote Wallace. "Four of these were located together on the breechblock." Another advantage was that Sailors could change out an overheated barrel in about 30 seconds. Even though the Oerlikon 20 mm had an effective range of 1,500 meters, most gunners were unable to take advantage of its full potential.

One of the biggest limits was human biology. Due to limited inter-pupillary space within the human eye, gunners could only judge distances accurately out to 400 yards. Wallace noted that this - and the limits of early gunsights -  forced gunners to aim with their tracers like a stream of water. As a result, accuracy suffered at extended ranges. In order to take full advantage of the gun's range, the Navy needed a new gunsight to make its firepower count. Luckily, Charles Stark Draper's instrumentation laboratory at Massachusetts Institute of Technology had just the solution. Nicknamed "Doc's Shoebox," the Mark 14 gunsight adjusted the elevation and lead of a targeting reticle through a reticle on two mirrors powered by an air-driven gyroscope. One compensated for movements in train - the other for elevation.
Doc's shoebox
Photo By: NHHC
VIRIN: 190319-N-ZV259-0286

Gun crews held the reticle on target and the gun barrel or barrels would quickly and continuously move in the right direction to permit the target to be hit. When mounted to the gun, the Mark 14 made the Oerlikon 20 mm a killer. According to an anti-aircraft summary published after the war, the Oerlikon 20 mm accounted for 617.5 enemy aircraft shot down between 1941 and 1945.

While it started the war as the most important anti-aircraft weapons system, within two years the Oerlikon 20mm was surpassed by the 5-inch gun and the 40 mm Bofors. Their heavier firepower was needed to combat increasingly fanatical Japanese suicide attacks. Nevertheless, the Oerlikon 20mm continued serving until well after the war. In many ways, the Oerlikon 20 mm was the right tool at the right time. While it was eventually phased out of service, the concept of a close-range defense for ships never disappeared. Advances in radar and computing systems allow just one Phalanx Close-In Weapons System (CIWS) to replace dozens of Oerlikons. And, like the Oerlikon, the CIWS fires the venerable 20 mm round. To learn more about the Oerlikon 20 mm watch out our Artifact Spotlight below: [embed]