Oerlikon 20mm AA gun

The Oerlikon 20mm anti-aircraft gun is one of the most successful and widely produced autocannons of the 20th century.

Developed originally from a World War I German Becker design, the weapon was refined and mass-produced in Switzerland by Oerlikon Contraves. It went on to serve as a definitive close-in air defense weapon for both Allied and Axis forces during World War II.

Key Technical Specifications
The Oerlikon relies on a unique Advanced Primer Ignition (API) blowback mechanism, which fires the cartridge while the bolt is still moving forward. This reduces the heavy recoil forces and allows for a lighter overall structure.
Caliber: 20 mm (typically utilizing 20×110mm RB ammunition)
Operation: API blowback, open-bolt, air-cooled
Cyclic Rate of Fire: 450–650 rounds per minute (theoretical)
Practical Rate of Fire: 250–320 rounds per minute (accounting for magazine swaps)
Ammunition Feed: 60-round top-mounted detachable drum magazine
Muzzle Velocity: ~820 to 1,050 m/s (2,725 ft/s)
Effective Range: ~914 to 1,500 meters (Vertical: ~4,500 feet)
Traverse & Elevation: 360° manual traverse; elevation ranging from -15° to +90°

World War II Impact and Legacy
The Oerlikon achieved its greatest fame on the decks of naval warships, serving as the primary short-range shield against enemy dive bombers and torpedo planes.
• The US Navy's Shield: The weapon was adopted massively by the United States Navy. Between December 1941 and September 1944, Oerlikon guns were credited with shooting down 32% of all Japanese aircraft downed by the US Navy. A single Essex-class aircraft carrier could bristlingly feature over 50 individual Oerlikon mounts.
• Global Production: More than 124,000 units were built in the United States alone (via manufacturers like Hudson Motor and Pontiac), while Great Britain built roughly 55,000. Germany utilized them under the designations FlaK 28 and FlaK 29.
• Evolution Against Kamikazes: By late 1944, the Oerlikon's stopping power proved insufficient against Japanese Kamikaze suicide attacks. Because a 20mm round would shred an aircraft but might not disintegrate it mid-air, it was increasingly replaced or supplemented by the heavier 40mm Bofors gun.
Despite being phased out of frontline roles after WWII, variants stayed active for decades. The British military deployed them as late as the 1982 Falklands War, and modern remote-controlled derivatives, such as the Rheinmetall Oerlikon Searanger 20, continue to be manufactured for contemporary naval defense.

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