WWII Tech Pubs Briefing

Written by
Frank Baldwin

Under constant air attack, troops of the BEF fight back with anti-aircraft artillery and even rifles while waiting to embark from the beaches of Dunkirk in late May 1940.

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Overview

The 40-mm (1.57-in) L60 Bofors gun is a remarkable weapon, still in service over 70 years since first designed. Both Allies and Axis deployed the gun in World War II.

Aktienbolaget Bofors-Gullspang, a company to become known as AB Bofors, was founded in 1873 in Bofors, Sweden, a town west of Stockholm. Bofors had grown into a major armaments manufacturer by the end of World War I. After the Treaty of Versailles, the German firm of Krupp acquired significant shares in Bofors and exchanged with the Swedes licences and patents so that Bofors could manufacture Krupp products while Krupp itself was forbidden to do so under terms of the Treaty. German engineers worked with Bofors, sharing many ideas before the Swedish government ended the arrangement in the 1930s. Out of this collaboration, the Swedes developed the 40-mm anti-aircraft (AA) gun and the Germans engineered their famous 8,8cm Flugabwehrkanone (Anti-Aircraft Cannon; FlAK) 18, 36, and 37.

The starting point for Bofors’s 40-mm gun was a Swedish navy requirement for a 20-mm (0.79-in) automatic AA cannon. Initial doubts about the effectiveness of small-calibre cannons grew until, in 1928, the Swedish navy funded the development of a 40-mm gun in an attempt to build an AA gun that would guarantee a kill with each hit. A prototype appeared in 1932 as a ship-mounted weapon with a jacketed barrel. A revised version with a water-cooled jacket was introduced in 1934. It had a barrel length of 56 calibres - often terminologically approximated to a 60-calibre length weapon, hence called a 40/60. This fine weapon attracted immediate attention from many armed forces, which were beginning to upgrade their air defences. The 40/60 initiated a new air defence category between light and heavy AA guns. It combined a heavier round than the 20-mm, with greater effective range and a mounting capable of fast traverse and elevation/depression for engaging low-flying aircraft. Most foreign delegations expressed more interest in a mobile field mounting than a naval mount. Bofors produced the Model 1934 (abbreviated M34) to meet their needs.

The M34 was the definitive weapon from which many of the variants were derived. The field carriage replaced the pedestal mount of the prototype with a lower mounting that featured trunnions to the rear. This mounting stored all systems used to control the gun. Twin equilibrator springs supported and balanced the muzzle preponderance. The whole of the top carriage, including the gun, rested on a bottom plate that also supported the platform frame assembly, which provided most of the working space for the crew in action. The bottom plate, in turn, rested on a cross-shaped field carriage. The fore and aft arms of the field carriage were formed from two box-section girders; folding outriggers to the left and right completed the cross. Screw jacks and levelling bubbles at the ends of the arms enabled the platform to be correctly leveled. The field carriage also contained front and rear balancing gear to aid in raising and lowering the carriage.

For transport, the M34’s field carriage used a twin-axle, four-wheeled arrangement that was normally detached before the gun went into action, although it was possible to fire the gun on wheels in an emergency. The front axle was steered with an Ackerman steering system operated via the draw bar. While in transit, the gun’s barrel was clamped to the rear axle.

The two gun layers sat to either side of the central body. The layer on the right operated the traversing mechanism, while the layer on the left operated the elevation mechanism and fired the gun with a pedal. The original Model 1934 had simple optical reflex sights mechanically coupled to a simple mechanical computer, the Bofors Course and Speed Sight. This sight let the layers correct for targets flying up to 350 mph (563 km/h), but it was a complicated system that took time to learn and required careful calibration. The carriage also contained a seat for the commander, close to the sight control unit. All fire orders were transmitted by voice.

What looked like a unified gun barrel and body was actually two separate items; a gun and a breech casing containing the loading system. The gun was made up of the 56-calibre-long air-cooled barrel, the breech ring, and the breech casing. The gun had a vertical wedge breech. The breech mechanism could be operated by hand but was usually operated by the gun recoil. A clockwork spring, actuated by the loading of a round, assisted opening and closing. The first round was loaded manually with a lever to the left of the breech casing and subsequent rounds were loaded by the recoil action, which also forced back the firing mechanism. The automatic loading system on top of the breech casing held three feed guides, each capable of holding one four-round clip. The automatic loading system removed clips and fed one round at a time to a loading tray. Once on the loading tray, a round was rammed automatically, and its case rims tripped the extractors that allowed the breechblock to rise, assisted by springs. Pressing the trigger pedal initiated the fully automatic loading and firing sequence. The loader or loaders stood or sat on the platform to feed ammunition clips into the loading assembly. A trained crew could change the barrel in under a minute.

Maximum theoretical rate of fire was 140 rounds/min but in practice, allowing for reloading, teams would achieve 80 rounds/min. At first, the M34 fired a high explosive/tracer (HE/T) round comprised of a 1.0-kg (2.2-lb) projectile fitted with a percussion fuse, a tracer that burned for 7-10 sec, and a self-destruct element that was usually initiated by the tracer. The round weighed 2.15 kg (4.74 lb) in total. Later ammunition included HE without tracer, drill, and armour piercing (AP) rounds.

The first orders for the 40-mm Bofors gun came from the Polish navy, Austrian army, and Belgian army, which all ordered small quantities. In 1935, Poland and Hungary made arrangements to licence production, and in 1936, orders began to arrive from increasing numbers of countries in larger and larger quantities, including 72 for the Dutch East Indies and 203 for the British Army.

The Polish State Arsenal produced 306 40-mm guns for the Polish army, as well as many that were exported to Britain.

In Hungary, the 40-mm gun was produced by MAVAG (Hungarian State Railways). The Hungarian armed forces deployed at least 767 guns; of these, 135 were mounted on 40M Nimrod self-propelled (SP) mountings, and a very few were mounted in Messerschmitt Me 210Ca-1 twin-engined aircraft. Hungary also exported guns to Finland, Great Britain, Norway, China, and Latvia. During World War II, the Hungarians provided at least 262 guns and 735 spare barrels to the German armed forces. The Hungarians added some technical developments to their Bofors, and were the first to use them in combat in conjunction with radar fire control. In one engagement against the Soviets at the River Tizer in 1943, Hungarian radar-controlled guns brought down all 25 attacking Petlyakov Pe 2 aircraft.

The Nimrod anti-aircraft tank was a Hungarian 38M Toldi chassis with a Bofors gun mounted in a high open-topped turret overhanging to the rear. The gun protruded through a vertical slot in the curved turret front. The turret accommodated a commander, two layers, and a loader. The gun was traversed and elevated manually. The prototype was produced in October 1941, and the Nimrod saw service from 1942 onwards in both AA and, less successfully, anti-tank (AT) roles. This was the first armoured self-propelled AA mounting of the Bofors gun.

Norway ordered three single-barreled naval guns from Sweden in 1936, and started licensed production of the M34 in 1940. After the invasion of Norway, the Germans took over production.

Belgium ordered eight guns in 1935 and that country’s Fabrique Nationale (FN) established a production facility for export. France ordered 34 guns in 1937 and acquired a further eight by 1940, which were all used in the AA defences around Paris. France had a licence to manufacture the gun, but did not produce them.

Although Germany initially expressed no interest in the Bofors gun, Austria acquired a licence to build 132 of them. By Mar. 12, 1938, the date of the Austro-German Anschluss (unification), factories had completed 24, nearly finished another 26, and had parts made for the rest. After the union, production temporarily stopped to produce army artillery pieces instead. Although the Luftwaffe (German air force) wanted the Austrian Bofors guns, German planners waffled for year over whether to ignore the guns, complete them for the Luftwaffe, or complete them for export sales. Having finally decided to press the guns into German service, on May 1, 1939, the Germans published a manual for what they would call the 4cm FlAK 28 (Bofors). The Austrian Bofors guns used Goerz sights.

By July 1940, the Germans had captured a large stock of 40/60s and the factories that produced them under licence. The only factory in which the Germans kept the Bofors gun in production was the Norwegian Kongsberg Våpenfabrikk, renamed Waffenfabrik Kongsberg. On Oct. 1, 1942, the Luftwaffe reported 340 40/60 Bofors guns in service. By June 1944, it would obtain another 94 new and 234 repaired guns.

The Kriegsmarine (German navy) also used these guns, and had 247 of them in coastal emplacements and on auxiliary ships on July 1, 1941. It ordered 800 more on Dec. 1, 1942, but the Norewgian and Hungarian factories could not meet the demand and the Kriegsmarine had received only 578 by July 1944. Some 60 late-model Schnellboote (Speed Boats; S-Boats) torpedo boats used a 4cm FlAK 28 (Bofors) as the main gun and even some bigger vessels, like the heavy cruiser Prinz Eugen, replaced their 3.7-cm guns with the Bofors.

The Soviet Union produced a 37-mm (1.46-in) gun that was not strictly a Bofors gun, but was obviously very closely based on the Bofors original. The Soviet design, the M1939, started with a Bofors design for a 25-mm (0.98-in) gun that was itself a scaled-down version of the Bofors M34 and which went into limited production in 1940. Soviet technicians produced their own scaled-up version in a 37-mm calibre, which went into production in 1939. The Soviet gun had an almost identical mechanism, but accomodated less refined manufactured parts. It used a more basic carriage and a simpler sighting system. The ammunition was less lethal but the Soviets were able to obtain a higher cyclical rate of fire. The M1939 was made in large quantities during and after World War II, and proved to be rugged and effective. It was also copied by China as the Type 55, and is still in use.

In the mid 1930s, the British were looking for an AA gun that could tackle the problem of high-speed low-level aircraft. Their first choice was the Vickers 2 pr (two-pounder) AA gun, originally designed for naval use. However, it was too large and heavy for use anywhere but in static defences. The British Army admired the Bofors 40-mm’s greater mobility and firepower and felt it would work with a new remote-control system then under development (the Kerrison Predictor). In April 1937, the British placed a first order for 100 guns, and followed this with three more orders, to Sweden, Hungary, and Poland. The Bofors gun was clearly a superior weapon to the 2 pr, which eventually returned to naval use only. Great Britain and Commonwealth countries also licensed manufacture, with main production by Nuffield (Coventry, England), Dominion Bridge (Vancouver, B.C. and Hamilton, Ont., Canada), and at Australia’s Ordnance Factory (Maribyrnong, Australia). Nuffield produced the first British-built gun in June 1939.

The first British-built guns took 2,420 man-hours to build, but the British made major changes to the Bofors design to simplify production and reduced production time to 1,500 man-hours. Changes included a simplified carriage with tubular steel legs that inserted into holes on the central frame, and elimination of the spirit levels, the balancing gear, and the Course and Speed Sight. The British redesign also included a shield, and was known as the Mk II Carriage.

The carriage was further drastically lightened for airborne use. The Mk IV platform enabled all four legs to be removed for stowage inside a General Aircraft Hamilcar glider. The Canadian two-wheeled Bantam carriage was a three-legged girder carriage on a single axle that could be towed by a Jeep or carried inside a Douglas C-47 transport aircraft.

The mountings used electrical power controls powered either by a generator or mains. The Kerrison Predictor, an electro-mechanical computer, would transmit fire controls to the gun depending on initial settings of range, speed, and angle.

While the British retained the original Bofors Course and Speed Sight on the guns delivered directly from Bofors – and called it the Polish Sight – on British-built guns, they installed the simpler Forward Area Sighting System, which used a wheel-like vertical foresight, a gate foresight, and crosswire rear sights. Simple range settings and aiming off could be made on the sights themselves.

The early Forward Area sights were replaced from 1943 by cartwheel foresights with three aim-off rings for 100, 200, and 300 mph (161, 322, and 483 km/h). Originally, these were fixed sights on a common cross bar. In 1944, a new system using the same sights was introduced. This was the Mk IV or correctional sight, known as the Stiffkey (pronounced “stookey”) sight, named after a firing range of the same name. A crewman called the sight operator operated the Stiffkey system on the platform. The operator used a ratchet bar to offset the cartwheel sights the layers looked through according to a target’s speed. All the layers had to do was to keep the target in the centre of the sight.

The British also deployed the Bofors gun on two SP mounts. Originally, there was little British interest in SP AA guns. Some Nuffield engineers decided that they could beef up Home Guard units formed from the Nuffield workforce with mobile Bofors guns on converted Morris Commercial Quad artillery tractors, also produced by Nuffield. After Nuffield demonstrated these SP installations to various army authorities, the British Army adopted the gun in time for the 1944 Normandy campaign. It is claimed that one demonstration was an unscheduled and uninvited demonstration at a display on Horse Guards Parade, where the civilian Home Guard demonstrated that they could bring their guns into action much faster than the regular army with its towed equipment.

The Nuffield carriage was called the Platform Carrier Morris 40-mm AA Mk I. This was the Morris Commercial C9/B 30-cwt (1500-kg) 4x4, a development of the earlier Morris C8 Field Artillery Tractor (FAT) that had been used in the earlier Home Guard conversions. The chassis mounting was specially developed for the vehicle. The mounting and cab were completely open with no cover for the crew except for some canvas weather shields. The truck’s rear axle could be locked to its frame so that the gun could be safely fired with minimal preparation. However, for use over prolonged periods or under predictor control, the crew would lower four jacks onto cones and base plates placed on the ground. Two of the jacks were at the sides, and the others at front and rear. The vehicle sat a driver and three men and could store 120 rounds. A similar vehicle carried a No.3 (Kerrison) Predictor, and had a driver and a crew of two. It also had storage space for a further 192 rounds.

In early 1944, several obsolete Crusader III tanks were converted into AA tanks. The Crusader III AA Tank Mk I mounted a single Bofors gun in a tall open-topped turret. The Bofors gun on this vehicle used a special type of autoloader. Spent cartridge cases were deflected forwards and upwards. The vehicle had a detachment of four, a gun commander, layer, loader, and driver. The layer controlled the powered mounting with a joystick.

The Bofors gun was also used extensively by the Royal Navy. There were five naval marks of air-cooled guns and two of water-cooled; water-cooled guns were made at the Royal Ordnance Factory (Nottingham, England). The naval mountings were generally simplified versions of the land mountings adapted for shipboard use. On small ships, elevation and traverse was by hand. Larger ships used power controls and centralised laying. The most advanced wartime mounting was the twin Bofors Mk IV Hazemeyer mounting which had all fire control equipment, including radar, on the mounting, and which featured a cross-roll device on the cradle that moved at right angles to the normal fore and aft mounting for better stabilisation. The Hazemeyer was designed by the Dutch firm of the same name – a Siemens Halske subsidiary – that delivered drawings and a sample mounting to Great Britain on a Dutch minelayer in 1940. The Hazemeyer, known informally as the “Hazelmere”, was probably a better design than the technology of the time could maintain. The follow-on mounting was the Mk V, a much simpler twin mounting which could use either a RP50 Metadyne remote power control (RPC) or local fire control.

The Royal Navy also used army guns, manned by Royal Artillery crews, in the naval role. These served on paddle steamers in the Thames Estuary, and on Defensively Equipped Merchant Ships. Some served on Royal Navy ships - one was lost on HMS Prince of Wales in 1941.

Many specialized mounts were designed or redesigned for the Bofors gun. One of the most complex wartime designs was the Twin 40-mm Bofors Stabilised Tachymetric Anti Aircraft Gun (STAAG) Mk II. This complex mounting carried a Type 262 radar, automatic target acquisition, a predictor, radar target tracking, stabilisation, a secondary rangefinder, and a spare generator. It had its own shield, and all controls and drives were hydraulic. As with the Hazemeyer, the mounting moved tri-axially, but the barrels on the STAAG used lateral deflection relative to the fire control equipment to aim ahead of the target. A marginal improvement in efficiency was achieved at the cost of added weight and complexity. One of the more printable names of the STAAG was the “many antlered beast”.

The Mk VII mounting was a much simpler and compact single mounting with a gyroscopic sight. This small variant was suitable for smaller craft from fast patrol boats upwards. One man layed the gun and another man loaded it. The Canadian-built Boffin mount was a conversion of an Oerlikon twin 20-mm Mk VC mount to accommodate a single 40-mm gun. It featured hydraulic controls and a crew of three. It was mostly used by the Royal Canadian Navy.

In the late 1930s, the US Army and US Navy were developing their own AA guns. The US Army looked at buying a single Bofors gun in 1938, but an error made the purchase price appear to be roughly ten times the true cost, which dampened interest. In 1939, a US businessman visiting the Bofors range was so impressed that on his return, he persuaded the US Navy to order a twin-barrelled Bofors gun. Trials in mid 1940 against various other AA guns, including the US Army’s Colt-Browning 37-mm Antiaircraft Gun M1, proved that the Bofors gun was superior. Both the US Navy and US Army decided to purchase the Bofors gun.

The US Army phased out production of the 37-mm M1gun when Bofors production was under way. In the haste to get the Bofors gun into production, the Americans obtained plans and began to establish production facilities before reaching any formal agreement with AB Bofors. In the ill-planned rush, the US Navy obtained drawings in metric measurements from the Dutch East Indies that were then converted to American blueprint standards and handed to the York Lock and Safe Company in York, Pa. Meanwhile, the US Army obtained a Bofors gun and drawings that used Imperial units from Great Britain, and passed them to Chrysler. As a result of the differences, the initial production runs of US Army and US Navy Bofors guns had a complete lack of interchangeability.

Like the British, Chrysler designers re-engineered designs and processes to speed mass production. The required low tolerances of error and complex machining challenged mass production practices. For example, the gun’s breechblock started as a 34.5-lb (15.6-kg) oblong steel block. After 110 manufacturing operations, it emerged as a sculptured component weighing 8.8 lb (4.0 kg). It took 140 operations and 625 inspection gauges to turn a 265-lb (120-kg) steel billet into a 105-lb (48-kg) breech ring. Another consequence of the US haste was a long contractual wrangle with AB Bofors over the unlicensed manufacture, sale, and disposal of Bofors guns, which was only resolved in 1957.

The US Army had only one version of the Bofors gun, the 40-mm Automatic Gun M1, which was normally installed on the 40-mm Gun Carriage M2 although it was also fitted to the Airborne Gun Carriage M5, the equivalent of the British and Canadian single-axle carriages. For fire control, the US Army adopted the Sight Computing M7 and M7A1. Fully predicted fire control was achieved later with the Director Antiaircraft M5, M5A1, or M5A2. The US Army experimented with self-propelled mountings of the Bofors Gun on the M3 and M5 Stuart Light Tanks, as well as halftracks, before testing the gun on the chassis of the T24 Light Tank (the precursor of the M24 Chaffee Light Tank). This mounted two 40-mm guns in an open turret with a frontal shield. The US Army designated this combination the T65E140-mm Gun Motor Carriage, and it entered production in August 1944 as the M19 Twin 40-mm Gun Motor Carriage.

The USS Coghlan (DD-606) on July 1, 1942 was the first front-line American ship to deploy the Bofors gun. US Navy production concentrated mostly on twin water-cooled mountings, which found their way onto almost all types of US warship. The usual mounting was the Mk 1 Twin Mount, which mounted its two barrels close together. The gun layers occupied their standard positions, while the loaders moved to a platform to the rear. Mountings were power-controlled. Usually, a combination of the lead-computing, relative rate Mk 14 gunsight and Mk 51 director carried out fire control. The Mk 14 sight had originally been developed at the Massachusetts Institute of Technology for the 20-mm Oerlikon, but it was easily adapted for the 40-mm Bofors. A single-barrelled, air-cooled Mk 3 mount, at first manually trained but later power-driven, was mainly used on fast patrol boats.

The Mk 2 Quadruple Mount, the US Navy’s first quadruple Bofors mount, was completed in April 1942 and installed on the gunnery training ship USS Wyoming on June 22, 1942. The quadruple mounting replaced existing quadruple 1.1-in (28-mm) AA and twin Bofors mountings on many ships. The Bofors gun could more ably counter Japanese kamikaze attacks than 20-mm guns. The same mechanisms that controlled the twin mounts controlled the quads. Many destroyers replaced torpedo tubes with army Bofors guns, which had guard rails to prevent gunners from firing at their own ships.

By 1945, most of the larger US warships were festooned with Bofors guns. The USS Saratoga held the record for having the most: 23 quadruple mountings and two twin mountings - almost as many 40-mm guns as one could find in the entire air defences of Great Britain in March 1940. The US also supplied quadruple mountings to the Royal Navy, many on Lend-Lease escort carriers. The first Royal Navy ship to be so equipped was the light cruiser HMS Pheobe in June 1943. Other Royal Navy ships with quadruple 40-mm mounts included the Troop Ship RMS Queen Mary.

Ammunition for the Bofors is still produced or available from a dozen countries to this day.

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Variants

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Major Variants Cont.

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Major Variants Cont.

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Specifications

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