Hazelwood (DD 531).
The first Fletchers deployed to the Pacific, Nicholas and O’Bannon (as well as Chevalier), arrived with minimal anti-aircraft defenses—a four-barrel 1.1-inch cannon plus six 20mm single Oerlikons, two forward (top image at right) and four in the waist (second and third images), plus 0.50 cal. machine guns. Unfortunately, the 1.1-inch was unreliable, the 0.50 cal. were ineffective and the Oerlikons were useful only as a last-ditch defense, but in 1942–43 nothing better was available in the South Pacific. Thus, gunnery officers responded by begging, borrowing or stealing as many 20mm single as they could find from non-combatant ships or shore installations, mounting as many as 13 wherever they could improvise, e.g., on the fantail (fourth and fifth images) and on top of the pilot house.

As 40mm twins gradually became available in 1942 and ’43, they were mounted aft (four barrels total) and later amidships (six barrels), leaving forward-firing secondary defenses to whatever 20mm mounts were available. Ships that had not yet left the states for the war zone were outfitted with additional 20mm forward—one on a platform between the bridge and the “52” mount forward, and one or two on the flying bridge for high bridge ships, and even a third on some low-bridge ships.

When the 10-barrel 40mm configuration began appearing after mid-1943, with the two new 40mm mounts replacing all 20mm forward, the number of 20mm was reduced and standardized at seven—four in the waist and three together in a heart-shaped gun tub on the fantail (top photo).

In 1945, when 40mm quads replaced twins amidships in the final World War II anti-aircraft modification, the single 20mm were also replaced by twin mounts—four in the waist as before but two, rather than three, on the fantail, for a total of 12 barrels.