Bofors 40 mm gun

The Bofors 40 mm gun, often referred to simply as the Bofors gun, is an anti-aircraft/multi-purpose autocannon designed in the 1930s by the Swedish arms manufacturer AB Bofors. It was one of the most popular medium-weight anti-aircraft systems during World War II, used by most of the western Allies as well as by the Axis powers. A small number of these weapons remains in service to this day, and saw action as late as the Gulf War.

In the post-war era the original design was not suitable for action against jet powered aircraft, so Bofors introduced a new model of significantly more power, the 40 mm L/70. In spite of sharing almost nothing with the original design other than the calibre and the distinctive conical flash hider, this weapon is also widely known simply as "the Bofors". Although not as popular as the original L/60 model, the L/70 remains in service to this day, especially as a multi-purpose weapon for light armored vehicles, as on the CV 90.

Bofors itself has been part of BAE Systems AB since March 2005.

Bofors 40 mm gun
Class Vehicle
Type Towed Artillery
Manufacturer Bofors
Production Period 1932 - 2000
Origin Sweden
Country Name Origin Year
Sweden 1934
Country Name Operational Year Retirement Year
Sweden 1934 View
United Kingdom - UK (Great Britain) View
United States of America View
ManufacturerName Production From Production To Quantity
Bofors 1932 2000 View
United Defense 2000 2006 View
BAE Systems AB 2006 View

The Swedish Navy purchased a number of 2 pounder Pom-Poms from Vickers as anti-aircraft guns in 1922. The Navy approached Bofors about the development of a more capable replacement. Bofors signed a contract in late 1928. Bofors produced a gun that was a smaller version of a 57 mm (6-pounder) semi-automatic gun developed as an anti-torpedo boat weapon in the late 19th century by Finspong. Their first test gun was a re-barreled Nordenfelt version of the Finspong gun, to which was added a semi-automatic loading mechanism.

Testing of this gun in 1929 demonstrated that a problem existed feeding the weapon in order to maintain a reasonable rate of fire. A mechanism that was strong enough to handle the stresses of moving the large round was too heavy to move quickly enough to fire rapidly. One attempt to solve this problem used zinc shell cases that burned up when fired. This proved to leave heavy zinc deposits in the barrel, and had to be abandoned. In the summer of 1930 experiments were made with a new test gun that did away with controlled feed and instead flicked the spent casing out the rear whereafter a second mechanism reloaded the gun by "throwing" a fresh round from the magazine into the open breech. This seemed to be the solution they needed, improving firing rates to an acceptable level, and the work on a prototype commenced soon after.

During this period Krupp purchased a one-third share of Bofors. Krupp engineers started the process of updating the Bofors factories to use modern equipment and metallurgy, but the 40 mm project was kept secret.

The prototype was completed and fired in November 1931, and by the middle of the month it was firing strings of two and three rounds. Changes to the feed mechanism were all that remained, and by the end of the year it was operating at 130 rounds per minute. Continued development was needed to turn it into a weapon suitable for production, which was completed in October 1933. Since acceptance trials had been passed the year before, this became known as the "40 mm akan M/32". Most forces referred to it as the "Bofors 40 mm L/60", although the barrel was actually 56.25 calibres in length, not the 60 calibres that the name implies.

The gun fired a 900 g (2.0 lb) high explosive 40 × 311R (rimmed) shell at 2,960 ft/s (900 m/s). The rate of fire was normally about 120 rounds per minute (2.0 rounds per second), which improved slightly when the barrels were closer to the horizontal as gravity assisted the feeding from the top-mounted magazine. In practice firing rates were closer to 80–100 rpm (1.3–1.7 rounds per second), as the rounds were fed into the breech from four round clips which had to be replaced by hand. The maximum attainable ceiling was 7,200 m (23,600 ft), but the practical maximum was about 3,800 m (12,500 ft).

The gun was provided with an advanced sighting system. The trainer and layer were both provided with reflector sights for aiming, while a third crew-member standing behind them "adjusted" for lead using a simple mechanical computer. Power for the sights was supplied from a 6V battery.

In spite of the successful development, the Swedish Navy changed its mind and decided it needed a smaller hand-swung weapon of 13 mm-25 mm size, and tested various designs from foreign suppliers. With the 40 mm well along in development, Bofors offered a 25 mm version in 1932, which was eventually selected as the 25 mm akan M/32.

The first version of the 40 mm the Navy ordered was intended for use on submarines, where the larger calibre allowed the gun to be used for both AA and against smaller ships. The barrel was shorter at 42 calibers long, with the effect of reducing the muzzle velocity to about 700 m/s (2,300 ft/s). When not in use, the gun was pointed directly up and retracted into a watertight cylinder. The only known submarines that used this arrangement was the Sjölejonet-class boats. The guns were later removed as the subs were modified with streamlined conning towers.

The first order for the "real" L/60 was made by the Dutch Navy, who ordered five twin-gun mounts for the cruiser De Ruyter in August 1934. These guns were stabilized using the Hazemeyer mount, in which one set of layers aimed the gun, while a second manually stabilized the platform the gun sat on. All five mounts were operated by one fire control system.

Bofors also developed a towable carriage which they displayed in April 1935 at a show in Belgium. This mount allowed the gun to be fired from the carriage with no setup required, although with limited accuracy. If time was available for setup, the gunners used the tow-bar and muzzle lock as levers, raising the wheels off the ground and thereby lowering the gun onto supporting pads. Two additional legs folded out to the sides, and the platform was then leveled with hand cranks. The entire setup process could be completed in under a minute.

Orders for the land based versions were immediate, starting with an order for eight weapons from Belgium in August 1935, and followed by a flood of orders from other forces including Poland, Norway, and Finland. It was accepted into the Swedish Army the next year, known as the "40 mm lvakan m/36", the lower-case "m" indicating an Army model as opposed to the capital "M" for Navy.

Because of the labour shortages some of the Bofors 40mm factories were opened in Poland.

The Swedish Navy adopted the weapon as the m/36 in hand-worked single air-cooled, and power operated twin water-cooled version. A twin air-cooled mounting, probably hand-worked was also used by the navies of Sweden and Argentina and a twin air-cooled wet mounting was developed for Polish submarines.

Type Autocannon
Place of origin Sweden
Service history
In service 1934–present
Used by See users
Wars World War II,
Indo-Pakistani wars and conflicts,
Arab-Israeli conflict,
Korean War,
Indonesia-Malaysia confrontation,
Vietnam War,
Yom Kippur War,
South African Border War,
Falklands War,
Lebanese Civil War,
Gulf War,
Yugoslav wars
Production history
Designer Bofors Defence
Designed 1930
Manufacturer Bofors Defence (1932–2000)
United Defense Industries(2000–2006)
BAE Systems AB (2006 onwards)
Produced 1932–present
Variants See variants
Specifications
Weight L/60: 1,981 kg (4,367 lb)
L/70: 5,150 kg (11,350 lb)
Crew dependent on use
Shell Complete round: -
L/60 40×311mmR (1.57 in), L/70 40×364mmR
Caliber 40 mm L/60–70 (actual length varies from 56–70 calibers, based on model)
Carriage 522 kg (1,151 lb)
Elevation L/60: -5°/+90° (55°/s)
L/70: -20°/+80° (57°/s)
Traverse Full 360°
L/60: 50°/s
L/70: 92°/s
Rate of fire L/60: 120 round/min
L/70: 330 round/min
Muzzle velocity L/60: 881 m/s (2,890 ft/s)
L/70: 1,021 m/s (3,350 ft/s)
Maximum firing range L/60: 7,160 m (23,490 ft)
L/70: 12,500 m (41,000 ft)

End notes