Iconic Firepower: The 40mm Borfors

Mar 31, 2014 No Comments by

By Barrett Tillman

How often have we seen the film clips? A U.S. Navy task group steaming beneath the Pacific sun as Japanese aircraft race through a flak-speckled sky intent on immolating themselves against the gray steel vessels. All the while, twin and quad-mount cannon churn out a wall of 40mm shells to deflect or destroy the raiders.

Ironically, those guns originated with one of the least warlike nations on Earth. Dating from the 17th century, Bofors was a Swedish firm known for its steel and artillery. The Model 1936 40mm cannon became the basis for arguably the most famous antiaircraft weapon of World War II.

The Swedish Navy first purchased antiaircraft guns from Bofors in the 1920s but desired a stronger, faster-firing weapon. Subsequent development produced a prototype in 1931, adopted the next year as the 40mm M/32. Bofors were air- or water cooled, though the latter was preferred since a high rate of fire caused barrels to become overheated.

The Swedes, leaders in military exports, found foreign customers for the M/32 throughout Europe. The British Army purchased single-barrel models from Bofors’ Polish factory and the Royal Navy ordered its first batch in 1937, installing them aboard battleships and cruisers five years later. Eventually, the British produced about a dozen versions, mainly for naval use.

American reaction was slow at first, largely because the European weapons were produced in metric measurements. Additionally, early ammunition was not considered “bore safe” owing to twitchy fusing that could detonate a round at the muzzle or from careless handling.  However, the weapon’s utility was obvious and U.S. ordnance engineers modified both the design and production techniques.

The U.S. Army recognized the Bofors’ advantages and ordered thousands of single-barrel 40mm guns with Chrysler beginning production in 1940.

The U.S. Navy moved quickly to adopt the Swedish design, fielding the first experimental mounts in early 1942.  Over the next three years, production approached 40,000 units; by 1945, the Army was paying $9,500 per weapon.

The basic water-cooled gun weighed nearly 1,200 pounds; the army air-cooled model, usually on a wheeled mount, ran barely 1,000.

With a muzzle velocity of 2,700 feet per second, maximum range of the 1.57-pound projectile was 11,200 yards at 40 degrees elevation, with effective altitude rated at 6,000 feet. The Navy shell contained a bursting charge of 2.4 ounces, sometimes enough for a solid hit to inflict fatal damage on a single-engine aircraft. The barrel life was reckoned at 9,500 rounds, and the Navy ordered approximately two spares per gun.

The Bofors was strip-fed with four rounds per clip, two clips capacity per gun. Coordination between the two loaders per barrel was essential to maintaining a high rate of fire, with a typical single-barrel cyclic of 120 rpm but upwards of 160 was possible when serviced by two beefy, well trained crewmen. The American 40mm round weighed 4.75 pounds, so a four-round clip ran nearly 20 pounds.

The naval guns were select-fire weapons capable of semi or full-auto mode, which is why some film clips show a two-gun mount firing one barrel single-shot.  Firing was accomplished by a foot pedal for each barrel, and the pedal(s) had to be kept depressed for sustained fire.

Naval fire control usually was provided by the Mark 51 director built around the Mk 14 gyroscopic sight, proven effective to 3,000 yards or 1 ½ nautical miles. Tracers allowed the director operator to “lead” or “lag” the computing sight as needed, depending on target speed and angle.

The Bofors manual cited a 10,000-yard maximum range or five nautical miles, but fusing usually was set to detonate rounds at about 5,000 yards to reduce friendly casualties. Time of flight to 4,500 yards was 10 seconds, and an inbound suicider making 250 knots covered that distance in barely 30 seconds. Therefore, a thick barrage of semi- and automatic fire was crucial to defending a ship.

During the war, the Navy built more than 2,400 quad mounts and nearly 10,000 each twin and single mounts. Costs were proportionate, at $67,500 for a quad and nearly $44,000 for a twin. However, the size and complexity of Bofors mounts required close coordination between the Navy’s Bureau of Ordnance and Bureau of Ships to accommodate the electrical and hydraulic plumbing.

The expanding two-ocean navy needed all the mounts the nation could produce, as an Essex class carrier often deployed 10 or more quads and Iowa class battleships of 1943-1944 were built with 20. The nine Independence class light carriers employed both dual and quad mounts.

Usually, a quad 40 was crewed by 11 men including a mount captain, pointer (controlling elevation), trainer (controlling azimuth), a first loader for each barrel, and second or other assistant loaders/handlers to move ammunition from ready-storage shelves in the mount and from below-decks magazines to the deck. Twin mounts generally required seven men. The director usually was manned by a pointer and a range setter.

The 40mm was reasonably fast for “point and shoot” scenarios, elevating at 24 degrees per second and traversing 30 degrees per second. However, wide-angle shots were relatively rare because a bomber or kamikaze attacking a specific ship normally presented a low or no-deflection target aspect.

Perhaps the best example of the Pacific War’s increasing antiaircraft requirement was USS Enterprise (CV 6), “The fightingest ship in the U.S. Navy.”  In 1938 she was commissioned with eight 5-inch guns, four 1.1 inch mounts, twenty-two 20mm cannon, and two dozen .50 calibers. But combat experience in 1942 showed a need for more defensive armament with a greater volume of fire.

Prolonged air attacks required prodigious amounts of AA ammunition. During the day-long Battle of Santa Cruz in October 1942, Enterprise consumed more than 50,000 rounds of AA ammo including 4,000 rounds of Bofors to claim 30 shootdowns.

Concluded historian Steve Ewing, “We’ll never know for obvious reasons, but Enterprise gunners shot down more planes at Eastern Solomons (August 24) in 15 minutes and 25 minutes at Santa Cruz than did the majority of all battleships, carriers, cruisers, and destroyers throughout the entire war.”

After “The Big E’s” 1943 upgrade, she emerged with six quad 40s, eight twin mounts (40 barrels), and 50 20mm Oerlikons.

The 40mm was effective but it seldom provided a one-shot kill.  The projectile was too small to accept the variable-time (VT) proximity fuse that made 5.38 inch shells so deadly, meaning that 40mm direct hits were required. Nonetheless, Bofors guns were credited with half of enemy aircraft splashed by shipboard gunfire from October 1944 through January 1945.

In a dramatic reversal of prewar doctrine, battleships became supporting players. Their size and speed permitted them to provide intense screening fire as escorts to carriers, hence the heavy AA emphasis on America’s last battlewagons, the four Iowa class ships.  Nonetheless, the Navy’s desire for more VT fuses hastened the Bofors’ postwar exit in favor of rapid-fire three-inch guns.

Meanwhile, the Bofors fought a truly world war. Apart from the U.S. and Britain, the type was adopted by Germany as the Flak 28 and Japan as the Type 5, adapted from captured British examples.

In U.S. service the Bofors remained a secondary weapon through the Vietnam era, though its utility largely ended in 1945.

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