One of the most widely used naval weapons of World War II, the 20-mm Oerlikon antiaircraft cannon was installed aboard virtually every U.S. Navy warship—from lowly PT boats to massive battleships—as well as in auxiliaries and armed merchantmen. It could be bolted down almost anywhere, had a high rate of fire, and could come into action very quickly. More than 88,000 20-mm Oerlikons, in both single and twin mountings, were manufactured in the United States between 1941 and 1945. Another 30,000 were manufactured by the British for use in the Royal and Commonwealth navies.
The Oerlikon’s origins stretch back to the Becker 20-mm automatic cannon, developed during World War I for use in German aircraft. After the war, the weapon’s patents passed to a Swiss firm. Meanwhile, in 1923, within a year of taking over the Swiss Machine Tool Factory Oerlikon, the Magdeburg Machine Tool Factory sent Emil Georg Bührle, a rising star in the parent company, to help straighten out the financially ailing acquisition. To boost sales, Bührle obtained the patents and manufacturing rights to the 20-mm Becker. Quickly placed into production, the weapon acquired an excellent reputation on the international arms market. Within a few years it was the company’s most important product.
Bührle was able to acquire a controlling interest in the Magdeburg Machine Tool Factory when it ran into financial difficulties in 1929. In an ironic quirk of history, in 1934 the firm was saved from bankruptcy—according to U.S. Navy records—by the Imperial Japanese Navy, which was seeking a more powerful gun for its aircraft and concluded a licensing agreement. The infusion of funds allowed the company to further improve the 20-mm gun, making it suitable for use as an antiaircraft weapon. In 1936 Bührle became sole owner of the firm, which he renamed the Machine Tool Works Oerlikon Bührle and Company.
About this same time, the Royal Navy was searching for a weapon that could be installed aboard merchant ships, trawlers, and minesweepers for defense against dive bombers and close-range air attacks, and in 1937 it selected the 20-mm Oerlikon. A big selling point was that the weapon needed no external power or cooling supply and could be bolted down in any available deck space. The gun could also be operated by almost anybody with only a limited amount of training.
In May 1940, shortly before the fall of France, Oerlikon engineer and representative Antoine Gazda arrived in America with manufacturing drawings of the 20-mm gun. His intention was to establish U.S. production of the Swiss weapon for the British Admiralty. Soon the British Purchasing Commission began negotiating with the subsidiary America Oerlikon Gazda Corporation to manufacture 2,000 of the guns in its Rhode Island plant, but could not get permission from the American government unless one of the U.S. armed services adopted the weapon.
The Admiralty quickly launched a campaign to convince the U.S. Navy to accept the 20-mm. Captain William H. P. “Spike” Blandy, the officer in charge of expediting antiaircraft gun production within the Navy’s Bureau of Ordnance (BuOrd), arranged for firing tests at the naval proving ground at Dahlgren, Virginia. All observers were favorably impressed with the gun, which then boasted a rate of fire of 450 rounds a minute, had an easily replaceable barrel, and was so robust that stoppages due to changes in elevation, cold weather, or rain were unlikely. Blandy, who even fired the gun himself, recommended its acceptance to the chief of the Bureau of Ordnance, Rear Admiral William Furlong. On 8 November, Arthur Purvis of the British Purchasing Commission wrote to Secretary of the Navy Frank Knox, urging him to adopt the Oerlikon as a standard weapon, and the next day Furlong approved its acceptance.
It took six months of bureaucratic haggling between the Navy, U.S. government, and the British Purchasing Commission before BuOrd could issue a contract to America Oerlikon Gazda under the Lend-Lease Act for 2,500 gun assemblies, 2,000 of which were slated for the British. The Hudson Motor Car Company and the Pontiac Motor Division of General Motors were also brought into the manufacturing picture.
The air-cooled 20-mm Oerlikon had some unusual features not found in other automatic weapons, including an open breechblock and a spring-recoil/counter-recoil system. It fired a 0.3-pound explosive shell supplied from a detachable magazine that held 60 rounds; expending all of them took just 7½ seconds.
The free-swinging mount on which the gun rested was originally equipped with open ring-sights for aiming. Pointing and training were controlled through a set of handlebars and shoulder rests. The gunner had to lead the target in both traverse and elevation to achieve hits. To aid in spotting, a tracer was usually included in every fifth round. By the middle of 1942, the open sights were being replaced by the Mark 14 gunsight (see “The Shoebox That Transformed Antiaircraft Fire Control,” December 2013, pp. 10–11).
Blandy wanted 20-mm Oerlikons eventually to replace every .50-caliber machine gun wherever the 20-mm’s greater weight could be accommodated. The mount, he wrote, “remains as steady as a rock, despite . . . its heavier projectile and muzzle velocity of 2,720 feet per second.”
Navy, Merchant Marine, and Lend-Lease demand for the 20-mm antiaircraft guns became so great that by the beginning of 1942 production at the three companies had been ramped up to 3,250 guns a month. More than 460,000 American men and women would eventually be involved in making the weapon during the war.
The importance of the 20-mm Oerlikon to the war effort cannot be overstated. Between December 1941 and September 1944, gunfire from the weapon was responsible for 32 percent of all Japanese aircraft downed by the U.S. Navy.
Lieutenant Robert Wallace, the automatic-weapons officer on board the USS Idaho (BB-42), left a vivid description of a 20-mm Oerlikon in action off Okinawa. A teen-age gunner opened fire on a kamikaze no more than one or two football fields away from the battleship while it tried to pass ahead of the ship. As Wallace observed, the young gunner “didn’t miss a beat in quickly getting a smooth track. He held his firing lever back for just 15 seconds, but that was enough. We could clearly see his tracers, in a full 90-degree deflection shot, seeming to curve into the cockpit. A ball of fire, the plane crashed harmlessly into the China Sea.”
By the end of 1944, the 20-mm Oerlikon’s importance had been overshadowed by the 40-mm Bofors’ greater hitting power and longer range. The enemy’s changing tactics and the Bofors’ increased volume and accuracy continued to reduce the comparative effectiveness of the smaller Oerlikon to the point where a large battery of the latter was neither necessary nor desirable.