Bofors 40 mm gun

40mm L/60

Development

The Swedish Navy purchased a number of 2 pounder Pom-Poms from Vickers as anti-aircraft guns in 1922. Looking for a smaller hand-swung weapon to complement the heavy Vickers, they asked Bofors A.B. to develop a 20 mm weapon based on a similar mechanism (and generally similar to the Vickers 1-pdr). Although this 20 mm design was not put into production, the Navy quickly soured on the 2 pdr, and approached Bofors in 1929 about the development of a much more capable replacement.

Bofors was initially reluctant, the Swedish Navy being a fairly small market, but the Navy eventually agreed to pay for the development of a prototype. Bofors responded with a gun that was, to some extent, a smaller version of a 57 mm (6-pounder) semi-automatic gun developed as an anti-torpedo boat weapon in the late 1800s by Finspong before Bofors drove them out of business. Their first prototype was in fact a re-barreled Nordenfelt version of the Finspong gun, using a new mechanism similar to the Vickers "machine gun" system using a moving bolt.

Testing of the prototype in 1929 demonstrated the major problem was feeding the weapon in order to maintain a reasonable rate of fire. A bolt that was heavy enough to handle the stresses of firing the large round was too heavy to move quickly enough to fire quickly. One interesting 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. Instead they experimented with a newer mechanism that simply "threw" the rounds into the breach from the rear without guiding them, the empty cases simply falling out to the rear when the breach was opened. This proved to be the solution they needed.

During this period Krupp purchased a 1/3rd 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. Nevertheless, many sources claim that the 40 mm design was in fact adapted from a Krupp weapon; it appears historians connected the development of the 40mm and German 37mm weapons without any supporting evidence.

By June 1930 testing with the prototype was complete, and Bofors reported that full-scale development could begin. A 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 finally ended in October 1933. Since acceptance trials had been completed the year before, this became known as the 40mm akan M/32. Most forces referred to it as the Bofors 40 mm L/60, although the barrel was actually 56.1 calibres in length.

The gun fired a 2 lb (900 g) high explosive 40 x 311R (rimmed) shell at 2,800 ft/s (850 m/s). The rate of fire was normally about 120 rounds per minute, which improved slightly when the barrels were closer to the horizontal as gravity assisted the feeding. In practice firing rates were closer to 80-100 RPM, as the rounds were fed into the breech from four round clips which had to be replaced by hand. The maximum attainable ceiling was 23,600 ft (7,200 m), but the practical maximum was about 12,500 ft (3,800 m).

The first version the Navy ordered featured a shorter barrel intended for use on submarines. The barrel was shorter at 42 calibres long, with the effect of reducing the muzzle velocity to about 700 m/s. When not in use, the gun was pointed directly up and retracted into a watertight cylinder. Interestingly the first order for the "full version" was made by the Dutch Navy, who ordered five twin-gun mounts for the cruiser de Ruyter in August 1934. These guns used two sets of layers, one to aim the gun, another to stabilize the platform that the gun sat on.

The Swedish Navy once again decided it needed a smaller hand-swung weapon of 13mm-25mm, and started testing 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 25mm akan M/32. For some reason the Navy found these more interesting than the 40mm version, and didn't place their own 40mm orders for some time.

Bofors also started the development of a suitable towable carriage which they displayed in April 1935 at a show in Belgium. The carriage caused something of a stir, as the gun could be fired from the carriage with no setup required, although with limited accuracy. If time was available 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.

The gun was also interesting due to its advanced sighting system. The trainer and layer were both provided with reflector sights for aiming, which a third crewmember standing behind them "adjusted" for lead using a simple mechanical computer. Power for the sights was supplied from a 6V battery.

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, Finland. It was only accepted into the Swedish Army the next year, known as the 40mm lvakan m/36, the lower-case m indicating an Army model as opposed to the capital M for Navy.

British versions

The British Army had first examined the weapon when they received a number of Polish-built examples in 1937 for testing, known as the QF 40mm Mark I, or Mark I/2 after a minor change to the flash hider. A license was soon acquired and the arduous task of converting the gun from metric to imperial measurements was started. While this was taking place they also made numerous changes to the design to make it more suitable to mass production - the original Bofors design was intended to be hand-assembled, and many parts were labeled "file to fit on assembly", requiring many manhours of work to complete.

Testing showed that aiming the guns was a serious problem against modern high-speed aircraft. Although the gun could be trained quickly, aiming accurately while doing so proved difficult. In order to address this, the British introduced a complex mechanical analogue computer, the Kerrison Director, which drove the laying electrically. A three-man team operated the Director simply by pointing it at the target and dialing in estimates for range and various atmospheric conditions, the gunners being reduced to simply loading the clips. Backup sights were then fitted, consisting of simple ring-and-post sights in place of the former reflector sights.

In this form, the QF 40mm Mark III (there was no Mk II), became the Army's standard light AA weapon, operating alongside their 3-inch and 90 mm heavy weapons. The gun was considered so important to the defence of England after the fall of France in 1940 that a movie, The Gun, was produced to encourage machinists to work harder and complete more of the guns. By the end of the war total production from British, Canadian and Australia factories was over 2,100, while US lend-lease examples added about 150.

In combat it was found that the Kerrison was simply too difficult to set up to use in many situations, as well as making logistics more complex due to the need to keep the electrical generator supplied with fuel. In most engagements only the "pancake sights" were used, without any form of correction, making the British versions less capable than the versions originally used by other forces. Eventually an anti-aircraft gunnery school on the range at Stiffkey on the Norfolk coast delivered a workable solution, a simple trapeze-like arrangement that moved the pancake sights to offer lead correction, operated by a new crewmember standing behind the left-hand layer. The Stiffkey Sight was sent out to units in 1943, arriving in Canadian units in the midst of the Battle of the Aleutian Islands. A final wartime change to the elevation mechanism resulted in the QF 40mm Mark XII.

The Army also experimented with various self-propelled AA systems, SPAAGs, based on various tank chassis. Changes to the breech for this role created the QF 40mm Mark VI, which was used on the Crusader to produce the Crusader III AA Mark I.

The Royal Navy also made extensive use of the Bofors. Their first examples were air-cooled versions quickly adapted to ships during the withdrawal from Norway. With the fall of the west in 1940 the Dutch minelayer Willem van der Zaan brought them their first example of a water-cooled gun on their Hazemeyer manually-stabilized mounting. Locally produced examples started arriving in 1942, known as the 40mm Mark IV for use in twin-mounts, or the 40mm Mark V for single mounts. The Navy ran through a variety of versions over the war, including the Mark VII to Mark XI.

The British also designed a much lighter two-wheeled carriage for airborne use, as well as a variety of mountings on older tank chassis for the SPAAG role.

In British service the Bofors found a highly specialist role. In the North Africa Campaign at El Alamein, they were used to fire tracer horizontally to mark safe paths for units through the German minefields.

US versions

The US Navy's Bureau of Ordnance purchased a twin-mount air-cooled example directly from Bofors, which arrived in New York on 28 August 1940. During that month another Dutch ship, the van Kinsbergen, demonstrated their mount to Navy observers. The gun was quickly chosen as the Navy's standard anti-aircraft weapon, and a production license was arranged with Bofors in June 1941, although by this time early production examples had already been built illegally using imported British plans. The resulting Mark 1 and Mark 2 weapons were intended for the left and right side of a twin mount, respectively.

The US Army had recently introduced a 37 mm gun of their own design, but found it to be of limited performance. Six British guns were imported, along with the Kerrison Directors, and proved to be superior in all ways. In order to supply both the Army and Navy with much greater numbers of the guns, Chrysler was brought in to be a major producer. Over the lifetime of the production, their engineers introduced numerous additional changes to improve mass production, eventually reducing the overall time needed to build a gun by half.

Captured examples

Germany had been banned from producing new weapons after signing the Treaty of Versailles at the end of World War I, but had extensive technology exchanges during the 1920s and early 1930s between Krupp and Bofors. The German 37mm Flak 28 was heavily based on the Bofors, and the Wehrmacht used a number of Bofors guns which had been captured in Poland and France. The Kriegsmarine also operated some guns obtained from Norway.

Japan captured a number of Bofors guns in Singapore and put them into production as the Type 5.

Continued use

Although the L/60 was later replaced by the L/70, the L/60 remained in front-line service well into the 1980s. In most cases these were the airborne versions, as a suitable replacement did not come along until the introduction of truly effective MANPAD missiles in the 1980s. L/60's are still used in the USAF's AC-130 gunships.

The Royal Navy used the L/60 into the 1990s, when it was replaced by a combination of missiles and smaller weapons that were more effective against missiles. The Canadian Navy followed their example, but removed the guns in the late 1980s when they were considered to be outdated. In a somewhat embarrassing episode, the Navy was forced to scour various military museums across Canada to re-equip their fleet during the Gulf War, as replacements had never been p

40mm L/70

By the end of World War II, jet aircraft had so increased the speed of attack that the Bofors simply could not get enough rounds into the air to counter the aircraft before it had already flown out of range. In order to counter these threats, the gun would have to have longer range and a higher rate of fire, thereby increasing the number of rounds fired over the period of an engagement. Bofors considered either updating the 40mm, or alternately making a much more powerful 57mm design, and in the end did both.

The new 40mm design used a larger 40 x 364R round firing a slightly lighter 870g shell at a much higher 1,030 m/s muzzle velocity. The rate of fire was increased to 240 rounds per minute, astonishing for such a large round. Additionally the carriage was modified to be power-laid, the power being supplied by a generator placed on the front of the carriage. The first version was produced in 1947, accepted in 1948 as the 40mm lvakan m/48, and entering Swedish service in 1951. Additional changes over the years have improved the firing rate first to 300 RPM, and later to 330.

Foreign sales started, as they had in the past, with Holland and Great Britain. In November 1953 it was accepted as the NATO standard anti-aircraft gun, and was soon produced in the thousands. The L/70 was also used as the basis for a number of SPAAG's, notably the US Army's infamous M247 Sergeant York, which proved to be unable to hit even hovering drone targets.

Breda of Italy uses the Bofors 40 mm L/70 gun in its anti-aircraft weapon system for the Italian army and navy. A newer development from Breda, the Fast Forty, has increased the rate of fire to an even more astonishing 450 RPM, normally equipped with a 43 round clip, or a 101 round clip for naval use.