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There were 49 of these 20 millimeter “Oerlikon” guns and 80 larger 40mm “Borfors” guns on BB-63 Battleship Missouri.
Throwing up a sheet of lead to keep the Zeros away.
Those open "gun tubs" were murder on the gun crews during battles. The losses among USMC and Navy AAA gun crews was just awful, accounting for almost half of all dead and wounded fleet wide.