The O’Brien was armed with eight 20MM Oerlikon antiaircraft cannons; two forward, two on the Aft 5” Gun Platform, and four surrounding the flying bridge, at each corner of the top of the house. These cannons were primarily used for defense.
Each was mounted in elevated, round steel gun tubs surrounded by a soft armor composite material of concrete, asphalt, and crushed rock, known as “plastic armor”. They fired a projectile that traveled 2,800 feet, or 853 meters, per second at a maximum rate of fire up to 450 rounds per minute. Each drum magazine could hold up to 30 rounds of ammunition. Unlike a machine gun that fired solid projectiles, a cannon would fire projectiles that would explode on contact.
During sustained firing, the barrel would eventually overheat. To overcome this, the outside of the gun tub was outfitted with two sections of pipe, one to hold a spare barrel and the other containing water to insert an overheated barrel for cooling.
It took several crewmembers to man a single 20mm cannon which included the need to pass ammunition from the ship’s magazine below and swap out the cannon’s empty drum magazines. During WWII, they were manned by the Naval Armed Guard and, when necessary, by trained Merchant Marine crewmembers.
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The tub and its gun is duplicated on the Port side. Immediately beneath the tub, is an armored storage area which also housed the experimental Mark 29 Anti-torpedo gear.