The Bofors 40 mm Automatic Gun L/60 (often referred to simply as the "Bofors 40 mm gun", the "Bofors gun" and the like, see name) is an anti-aircraft autocannon, designed in the 1930s by the Swedish arms manufacturer AB Bofors. The gun was designed as an intermediate anti-aircraft gun, filling the gap between fast firing close-range small calibre anti-aircraft guns and slower firing long-range high calibre anti-aircraft guns. For its time, the Bofors 40 mm L/60 was perfectly suited for this role and outperformed competing designs in the years leading up to World War II in both effectiveness and reliability.
It entered the export market around 1932 and was in service with 18 countries by 1939. Throughout World War II it became one of the most popular and widespread medium-weight anti-aircraft guns. It was used by the majority of the western Allies and some Axis powers such as Nazi Germany and Hungary.
In the post-war era, the Bofors 40 mm L/60 design was not suitable for action against jet-powered aircraft, so Bofors developed a new 40 mm replacement design with significantly more power—the Bofors 40 mm Automatic Gun L/70, also known under the generic name 'Bofors 40 mm gun'—which was adopted by many nations during the Cold War and was selected as NATO-standard in November 1953. The Bofors 40 mm L/60 would however continue to see service long after becoming obsolete as an anti-aircraft weapon due to the massive number of surplus guns from WWII, and a small number of Bofors 40 mm L/60 guns remain in service today. Some weapons saw action as late as the Gulf War and Yugoslav Wars.
Name The Bofors 40 mm Automatic Gun L/60 is known under a variety of names around the world. In common historical nomenclatures it is commonly known under the names Bofors 40 mm gun, 40 mm Bofors gun and Bofors gun. This mostly stems from Western Allied usage of the gun during World War II. For example, British anti-air batteries armed with the gun during the war received the nickname "The Bofors Boys". There were other guns by Bofors which also had these nicknames, the most common being the Bofors 40 mm Automatic Gun L/70, leading some to think they are the same gun.
Bofors itself never marketed the gun or its variants with a clear standardized name or abbreviation. In the vast majority of 1930s marketing material the gun is sold under a variety of descriptive names depending on the configuration, such as: "40 mm Automatic Field Gun L/60" to "40 mm Automatic A.A. Gun L/60 in Field Carriage" or "40 mm Automatic Naval Gun L/60" to "40 mm Automatic A.A. Gun L/60 in Naval Mounting". This theme transfers over to other languages as well: , , . Only the caliber of 40 mm and caliber length of L/60 are somewhat universal in branding.
Due to this the gun is often specified by the calibre length "L/60". Enthusiasts and experts usually call the gun "Bofors 40 mm L/60" or simply "Bofors 40/60" and the like. In some Bofors material the gun is standardized under the English name "Bofors 40 mm Automatic Gun L/60".
Calibre length Contrary to the name, the Bofors 40 mm Automatic Gun L/60 has an actual barrel length of 56.25 calibres – . Due to this, the gun can at times be found under the designation L/56. The L/60 calibre length – – refers to the length of the barrel and breech mounted as a unit. The gun has also been cited with a caliber length of L/62 on occasion. This refers to the length of the barrel – 2,250 mm – when equipped with the conical flash hider – ; amounting to a length of , or 62.5 calibres. Similarly, the shorter L/43 calibre length – – refers to the length of the barrel and breech mounted as a unit. The actual barrel length is – 39.25 calibres. With a flash hider or muzzle brake mounted the barrel length becomes or .
Development In 1922, the Swedish Navy purchased a number of 40 mm 2-pounder "pom-poms" from Vickers as anti-aircraft guns. The navy approached Bofors about the development of a more capable replacement, and Bofors signed a contract in late 1928. The company 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 Finspång. Bofors' first test gun was a re-barrelled Nordenfelt version of the Finspång gun, to which was added a semi-automatic loading mechanism.
Testing of the gun in 1929 showed that a problem existed with feeding the weapon 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 operate quickly enough to fire rapidly. One attempt to solve the problem used zinc shell cases that burned up when fired. However, that left heavy zinc deposits in the barrel and had to be abandoned. In the summer of 1930, experiments were conducted with a new test gun that did away with controlled feed and instead flicked the spent casing out the rear, after which a second mechanism reloaded the gun by "throwing" a fresh round into the open breech from the magazine. That improved firing rates to an acceptable level, so work on a prototype commenced soon after.
During that period, Krupp purchased a one-third share of Bofors. Krupp engineers began the process of updating the Bofors factories with 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, the gun was operating at 130 rounds per minute. The development needed to turn it into a weapon suitable for production was completed in October 1933. Because acceptance trials had been passed the year before, it 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 implied.
The gun fired a high explosive shell at from a 40 × 311 rimmed cartridge. The rate of fire was normally about 120 rounds per minute (2 rounds per second), which increased slightly when the barrels were closer to the horizon, because gravity assisted the feeding from the top-mounted magazine. Practical firing rates were closer to 80–100 rpm, because the rounds were fed into the breech from four-round clips which had to be replaced by hand. The maximum attainable ceiling was , but the practical maximum was about .
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-traversed weapon of 13 mm-25 mm size, and tested various designs from foreign suppliers. With the development of the 40 mm well advanced, Bofors offered a 25 mm version in 1932, which was eventually selected as the Bofors 25 mm M/32.
The first version of the 40 mm the Swedish Navy ordered was intended for use on submarines, because the larger calibre allowed the gun to be used both as an anti-aircraft gun, and against smaller ships. The barrel was shorter, at 43 calibres, which reduced the muzzle velocity to about . When not in use, the gun was pointed directly up and retracted into a watertight cylinder. The only known submarines that used that arrangement were the Sjölejonet-class boats. The guns were later removed when the submarines were modified with streamlined conning towers.
The first order for the "real" L/60 was made by the Dutch Navy, which ordered five twin-gun mounts for the cruiser in August 1934. Those 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 was displayed at a show in Belgium in April 1935. That mount allowed the gun to be fired from the carriage with no set-up required, although it had limited accuracy. If time was available for set-up, the gunners used the tow-bar and muzzle lock as levers, raising the wheels off the ground and so lowering the gun onto supporting pads. Two additional legs folded out to the sides, and the platform was then levelled with hand cranks. The entire set-up process could be completed in under a minute.
Orders for the land-based versions were immediate, starting with one for eight weapons from Belgium in August 1935, followed by a flood of orders from other forces including Poland, Norway, and Finland. It was accepted into the Swedish Army the next year, and known as the "40 mm lvakan m/36", the lower-case "m" indicating an army model as opposed to the capital "M" for the navy.
Licensed production Within a few years AB Bofors received a number of orders from foreign governments including Poland, Finland, Greece and Norway negotiated for licenses to produce the gun and its ammunition. By 1939 Bofors guns were in production across Europe for many armies in a bewildering arrangement of cross-deals. For instance, the United Kingdom took out a license, but was in such a hurry to re-arm with the Bofors gun that it also purchased some from Poland and Hungary. France wanted to set up a line but purchased guns from Poland. Poland meanwhile developed its own version of Bofors gun, contributing a lighter carriage (in the 40 mm armata przeciwlotnicza Bofors wz. 36) which was later adopted by the British.
The Swedish navy adopted the weapon as the m/36 in two versions: hand-worked single air-cooled, and power-operated twin water-cooled. 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.
British versions Army and RAF Regiment versions
The British Army had first examined the weapon in 1937, when it received a number of Polish-built examples for testing. They were known as the "QF 40 mm Mark I" (QF standing for "quick firing"). With a minor change to the flash hider, they were designated "Mark I/2". A production licence was acquired, and the gun was converted from metric to imperial measurements. Numerous changes were made to the design so that it was more suitable for mass production, because the original Bofors design was intended to be hand-assembled. Many parts were labelled "file to fit on assembly", requiring many man-hours of work to complete.
Testing showed that there was a serious problem with aiming the gun at high-speed aircraft. Although it could be trained quickly, aiming accurately while doing so proved difficult. In order to deal with that, the British introduced a complex mechanical analogue computer, the Kerrison Director, which drove the laying electrically. A three-man team operated the director by pointing it at the target whilst dialing in estimates for speed, range, and various atmospheric conditions. The director then aimed the gun using powered mounts, while a gunner loaded the clips. That eliminated the need for the lead-correcting reflector sights, which were replaced with a backup system consisting of a simple ring-and-post sight, known as a "pancake".