The Oerlikon 20mm Gun is an autocannon widely produced by Oerlikon Contraves during World War II, likely in higher numbers than any other antiaircraft gun of the war, with various models employed by both Allied and Axis forces.
These guns are open-breech, air-cooled and use a gas blow-back recoil system. This weapon has some unusual features not found in other automatic guns. When fired, the case pushes the breech back against the force of the barrel springs. The barrel does not recoil, the breechblock is never locked against the breech and is actually moving forward when the gun fires. This weapon lacks a counter-recoil brake, as the force of the counter-recoil is checked by the explosion of the next round of ammunition.
One of the largest advantages of this weapon from a maintenance point of view was that, unlike the contemporary Hispano-Suiza 20mm gun, the forces used to operate the automatic mechanisms were quite high, which meant that such factors as friction, insufficient lubrication, cold weather, different elevations, rain and the like were small in proportion to the operating forces involved and therefore unlikely to cause stoppages. In addition, the barrel on the Oerlikon gun could be changed in 30 seconds or less while it took about an hour to change barrels on the Hispano-Suiza.
Description [partially] lifted from NavWeaps.com
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Tier: 9-B single shot, higher in short bursts
Name: Oerlikon 20mm/70 Gun
Origin: Real World
Classification: Autocannon, Anti-Aircraft Gun
Attack Potency: Wall level single shot (42.5-101.5 kilojoules), higher in short bursts; up to 450 rounds per minute rate of fire
Speed: Supersonic (835–844 meters per second)
Durability: At least Street level+, at most Small Building level with biggest turret mount in terms of total destruction (comprised of up to 1300 kilograms of steel with a fragmentation energy of 35 megajoules)
Stamina: Barrel life of 9000 rounds
Range: Up to over 4 kilometers (AA ceiling of just over 3 kilometers)
Weaknesses: Generally ineffective against Japanese Kamikaze attacks.
Note: Similar guns of about the same bore (20mm) and period generally have performance equivalent to this one.