Bofors 40mm

We wrote briefly about the M42 Duster, and Craig provided a nice walkaround of this twin 40mm gun motor carriage. I guess we should discuss the heart of the weapon system, the Bofors 40mm anti-aircraft gun.

The 40mm Bofors L/60 was probably the most successful medium anti-aircraft weapon of the 20th century. Designed in Sweden in the 1920s and early 1930s, it was used on both sides of World War II, seeing service with the US, Britain, Germany and Japan. Indeed, the L/60 still serves with the US Air Force on board AC-130 gunships.

Originally designed as a single barrel, air cooled mount that was hand cranked and aimed over open sights, later models, especially those in US Navy service would be multiple mounts, often water cooled, with power traverse and elevation, and aimed by optical and later radar fire control directors.

As World War II began, and US entry into the war loomed, the US Army had designed its own 37mm anti-aircraft gun, but wasn’t entirely convinced of its capability. The US Navy at the time was equipping its ships with a quadruple mounted 1.1-inch machine gun mount. Both services were well aware of the threat posed by airpower. They were not at all blind to German successes using airpower both in Blitzkrieg on land, and in the anti-shipping role in the Atlantic.

The British began license producing Bofors in the years immediately prior to the outbreak of war. As US entry began to look more likely, they provided examples to the US, who quickly began to modify and produce it, even before a production license was granted by Bofors (a license would eventually be granted in 1941). Chrysler modified the design, mostly to suit mass production methods. Eventually, Chrysler would produce a staggering 60,000 guns.

In Army use, the most common mount was a single air-cooled  gun mounted on a simple four-wheel towed carriage. The gun fired a roughly 2 pound projectile out to a maximum effective range of 3,500 yards. The high-explosive projectile has a contact fuse. It was a large enough projectile that a single hit would bring down most fighters, and often many bombers of the day. Due to the small size of the projectile, it could not accommodate time fused shells common in larger guns like the 3” or 90mm anti-aircraft guns (nor could it accommodate the later proximity fuse).

40mm snow 1

We tend to have a mental picture of the 40mm being operated by gunners cranking hand controls to lay the gun. And while that was often the case, in US Army service, the gun was actually intended to be power operated and laid by a sophisticated optical gunfire director. Hitting a moving target like an airplane required leading the target, and determining that lead angle by eye is almost impossible. The rate of fire was so low that correcting by eye from observing tracers would take so long that the engagement would be over before the rounds were on target. Hence, the need for director control.

A M5 director was essentially an analog computer to determine the lead angle needed to engage a given target. By optically tracking the  range, elevation, bearing, and bearing change rate of the target, the director could determine where the gun should be fired for the rounds to hit. As a further refinement, the director could transmit that information to the gun mount as DC current. The gun mount, powered by an external generator, could be remotely controlled from the director, elevating and training without any input from the on-mount crew. The mount’s crew needed only to feed the ammunition to the gun, and depress the firing pedal.

40mm fire unit

Most Coast Artillery anti-aircraft battalions equipped with the 40mm gun in North Africa and Italy used the M5 director, but with the increase Allied air superiority in Western Europe, and the rapid pace of movement there, most post-Normandy units left the directors behind in an effort to increase mobility.

In naval usage, some smaller combatants such as subchasers used unpowered single barreled mounts (and frequently referred to them as “Army style mounts”). But by far, most Navy Bofors mounts were twin or quadruple mounts, powered in train and elevation, stabilized, and directed by the Mk 51 optical director. The Mk 51 was a simple one-man director that used a gyroscope to determine lead angle. One Mk 51 was normally devoted to each mount, but each director could direct the fire of up to six mounts to concentrate fires. In a pinch, the Mk 51 could even control the fire of 5”/38 guns.


As the Japanese Kamikaze campaign inflicted horrific losses on the US fleet the Navy realized that something with a little more oomph would be needed to destroy a determined attacker. Consequently, the 40mm Bofors was slated to be replaced by the rapid fire 3”/50 gun. Single mounts would replace twin 40mm mounts on a 2 for three basis, and twin mounts would replace quad forties on a similar basis. The 3”/50 was valued for its greater stopping power, and its ability to use proximity fuses.

But the 40mm would continue to serve on Navy ships for a great many years, and as seen by the M19 and M42 Duster, the Bofors 40mm would continue to enjoy a long career with the Army.

Eventually, the L/60 gun would be replaced in service by the L/70 40mm gun. Firing a more powerful round, with a longer range and greater rate of fire, the L/70 was the main weapon of the failed M247 Sgt. York Division Air Defense System (the guns were about the only thing on the program that worked). The L/70 remains in production, is capable of firing a wide variety of programmable fused ammo, and is used as the main armament on several modern armored vehicles.

10 thoughts on “Bofors 40mm”

  1. Were the actual guns on the M19 and M42 Navy style liquid cooled guns? The barrels look rather thick for air cooled guns.

    1. They’re air cooled. Compare the pics from the walkaround post to the barrels on the guns in the video in this post.

      The Army’s earlier 37mm AA gun was “water-cooled” in that between firings, you could stick a hose in the muzzle and breech and cycle water through it to cool it faster.

    2. Regarding the 37mm AA gun – it was developed at the close of World War I as an automatic weapon to replace the 37mm trench guns. John Browning, who we know better for machine guns and pistols, did the initial design work. Like all post-WWI designs, the M1 37mm followed a long evolution between the wars, first being “limited procurement” then full standard issue. What I always point out is the Army followed two lines developing 37mm projectiles – the AA gun and the aircraft cannon (P-38s and P-39 fighters) fired the same projectile. The 37mm AT gun and tank main gun fired a different round. There’s an R&D/procurement lesson learned along the way….

    1. I could probably work up something on the 3″/50, but the 3″/70 was a failure, and there isn’t a lot of info on it.

      Pretty much the only thing I’ve ever read about it was in Friedman’s design history of destroyers. I’ve never even seen one.

      Though just outside Kokomo, IN, there’s some guy that had a twin 3″/50 in a gunhouse sitting outside his farmhouse.

      Duck season must have been fun there.

  2. “Chrysler modified the design, mostly to suit mass production methods. Eventually, Chrysler would produce a staggering 60,000 guns.”

    Another point to make about the US 40mm Bofors and the original model is part interchangeability. In theory, some parts of the US weapon should work on the British or original Swedish (or Polish guns for that matter). Reality is that Chrysler re-engineered the weapon from the carriage to the muzzle. Parts are not parts, in this case.

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