Ian from Forgotten Weapons presents the 20mm Oerlikon automatic cannon – a mainstay of the United States Navy, used as a defense weapon during World War II. He looks at and discusses a few of the cannon’s characteristics and other aspects that only apply to such weapons, and not typical small arms. For example, he says in his caption, how do you cock a gun that has a 400 pound recoil spring? Or, what happens if you fire a high explosive shell into your muzzle cover?
The Oerlikon is a monstrous, fully automatic weapon. Here are four facts on them. First of all, these guns were used by both sides in World War II – Allied and Axis forces alike. The Germans, Italians, Americans, British and Japanese all used these cannons. Between December of 1941 and September of 1944, these were responsible for 32% of all Japanese aircraft that the US shot down.
Second, these are open bolt guns. There is no locked breech, despite this being a massive, 20mm cartridge. The way it works is called advanced primary mission, which is different from how it works on submachine guns. On those, there is a fixed firing pin that will fire a cartridge immediately upon chambering it. Here, the chamber is different. Ian explains how it works, and the positive and negative aspects of the system.
He also explains how to cock the cannon, given its massive size, weight and force – a cocking rope is needed, along with a pulley that will pull the charging handle.
Third, these are primarily naval guns. They had muzzle covers, but if you’re out at sea, and it’s spraying water and it’s below freezing, it’s not good. So they took out the muzzle covers, and tweaked how to load the drum, using just solid projectiles that were not highly explosive.
Lastly, these cannons could drop five magazines and not do any damage to the barrel. But at nine magazines, the barrel would be damaged. One advantage of the gun is that the barrel can be changed in thirty seconds.