The 40mm Bofors Gun

A quad 40mm Bofors gun crew in action.

One of the most effective anti-aircraft guns of World War II, the 40mm Bofors gun played a vital role in defending Navy ships. Originally of Swedish design, but obtained from the British, at the outset of the war the Bofors gun was not standard on U.S. Navy ships. This changed by 1942, when American designers updated the gun and improved the ammunition. The 40mm guns replaced previous ineffective guns and provided an upgrade by becoming one of the prime anti-aircraft batteries for the Navy, alongside the larger twin dual-purpose 5-inch guns and smaller 20mm guns.      

The Bofors guns were usually arranged in either dual or quad positions, and less commonly as a single barrel. The arrangement generally depended on the size or layout of a ship. For instance, USS Wisconsin (BB 64) had twenty quad 40mm mounts (80 gun barrels), whereas a destroyer escort might only have several  dual 40mm mounts. 

The gun crew consisted of eleven men for the quad guns, with two loaders needed for each barrel. The sailors worked as one unit under the gun captain, who was able to direct the gun quickly on a fast moving turret. The gun was improved when it was combined with the new Mk 51 gun director which enabled greater accuracy. Between October 1, 1944, and February 1, 1945, the Bofors/Mk 51 combination was credited with 50% of all Japanese aircraft shot down by U.S. Navy ships.

A painting showing a dual 40mm mount on an aircraft carrier from Task Force 58 in the Pacific. All the main carriers in this Task Force were built in Hampton Roads. Note the Mk 51 director in the foreground.

Quad mounts firing on an Iowa-class battleship

The Bofors gun enjoyed wide popularity during World War II as it was not only used by the Navy, but was utilized by other branches as well. Of course, the British also used it, but it is worth noting that both the Japanese and Germans used captured guns or tried to replicate their own when they could do so. They remained popular long after the war, and a few militaries still employ variations of the 40mm gun to this day.