This weapon traces its roots back to a 1918 Krupp design - the Bofors Company was partly owned by German interests until 1930 - but the finished product was entirely a Bofors design owing little or nothing to Krupp influence. During the late 1920s the Swedish Royal Navy approached Bofors a few times with requests to design a new 40 mm AA gun to replace the British 2-pdr. guns currently in service. Bofors showed little interest in this project as the size of any order from the Swedish Navy would be small and they did not see much possibility in foreign orders. However, the Navy persisted and finally offered to help fund the development costs. With this money in hand, Bofors started to actively work on a new AA weapon. The first Bofors test-bed prototype was finished late in the summer of 1930 and this fired three rounds using the automatic loader on 17 October 1930. Design and manufacturing of the first gun followed and single shots with it were fired on 10 November 1931. By 25 November 1931 trials had progressed to firing eight rounds in 7.58 seconds. This showed that the autoloader design concept would work, but more effort was needed before the prototype could meet the Navy's 130 rpm requirement. After the necessary improvements, official trials for the Swedish Navy took place successfully on 21 March 1932.

The basic design was further refined over the next two years, with 30,000 hours of drawing board work needed to produce 1,800 working drawings and 1,600 machine tool drawings that made up the improved 40 mm L/60 Model 1934. This model was exported to several countries but not adopted by the Swedish armed forces. What the Swedish Navy did order in 1934 was a short-barrel version for submarines that was to be stowed in a waterproof tubular container located in the conning tower or under the decking. This weapon was designated as as the 40 mm L/43 Model 1932, please see that datapage for details.

It was not until 1936 that the Swedish Army and Navy placed orders for a very slightly modified version and this became the 40 mm L/60 Model 1936 whose basic design was used for most guns produced during World War II by Bofors, the UK and the USA. Although the UK and USA halted production with the end of the war, Bofors continued to manufacture the 40 mm L/60 until 1954 when production was halted in order to concentrate on the more powerful 40 mm L/70 and 57 mm L/60 guns.

The British Army first showed interest in these guns in 1933 and placed an order for 100 of them in 1937. First Royal Navy shipboard use of air-cooled guns was in late 1941 aboard the battleships Prince of Wales and Nelson and on the cruisers Manchester and Erebus, although some ships had earlier been temporarily armed with Army air-cooled guns that had been "rescued" during the evacuation of the Norway invasion forces in 1940. The British water-cooled version was developed by copying the Dutch Hazemeyer mounting which had arrived in Britain in 1940 aboard the Dutch minelayer Willem van der Zaan. The first issue of locally produced water-cooled Bofors guns was to the Black Swan class sloop HMS Whimbrel in November 1942.

The total number of air-cooled guns built by Australia, Britain and Canada is not accurately known but was somewhere between 2,100 and 2,800 plus about 200 to 400 guns supplied from the United States. Water-cooled guns are better documented with 442 Mark IV and 342 Mark XI in service at the end of the war plus 786 water-cooled guns supplied by the USA. These USA weapons had been sent to Britain as a part of Lend-Lease or else were installed on ships refitted in USA shipyards.

The US Army was also interested in this weapon and tested a single air-cooled model in 1937. In 1940 the Chrysler Corporation agreed to begin manufacturing air-cooled guns utilizing British blueprints. The USN acquired many of these air-cooled guns during the war, although the quantity used was far less than that of the water-cooled guns.

The US Navy had a good deal of pre-war interest in this weapon and BuOrd purchased a sample of an water-cooled twin version from Bofors in early 1940. Bofors instead sent an air-cooled version and this arrived in New York from Sweden on 28 August 1940. During the same month, the Dutch escort vessel van Kinsbergen demonstrated these weapons and their Hazemeyer control system to USN observers in a test off Trinidad. The observers were not impressed by the control system, but were favorably impressed by the guns themselves. BuOrd formally obtained Swedish licenses in June 1941, although some manufacturing had actually started prior to that time. Terms of the license included a $500,000 payment for the manufacturing rights plus $100,000 for two Bofors engineers to help set up production. The two engineers were never sent, so as a result this $100,000 was not paid. Bofors delivered a complete set of metric drawings as part of their end of this contract.

It should be noted that the US Army and Navy considered the original Bofors Model 1936 design to be completely unsuitable for the mass production techniques required for the vast number of guns needed to equip the anti-aircraft batteries and ships of the US military. First, the Swedish guns were designed using metric measurement units, a system all but unknown in the USA at that time. Worse still, the dimensioning on the Swedish drawings often did not match the actual measurements taken of the weapons. Secondly, the Swedish guns required a great deal of hand work in order to make the finished weapon. For example, Swedish blueprints had many notes on them such as "file to fit at assembly" and "drill to fit at assembly," all of which took much production time in order to implement - there is a story that one USA production engineer remarked that the Bofors gun had been designed so as to eliminate the unemployment problems of the Great Depression. Third, the Swedish mountings were manually worked, while the USN required power-worked mountings in order to attain the fast elevation and training speeds necessary to engage modern aircraft. Fourth, the Swedish twin gun mounting supplied to the USA for evaluation was air-cooled, limiting its ability to fire long bursts, a necessity for most naval AA engagements. Finally, the USN rejected the Swedish ammunition design as it was not boresafe, the fuze was found to be too sensitive for normal shipboard use and its overall design was determined to be unsuitable for mass production.

US manufacturers made radical changes to the Swedish design in order to minimize these problems and as a result the guns and mountings produced in the USA bore little resemblance to their Swedish ancestors. For example, all but the earliest US guns were built to English measurement units rather than to metric units. To give one additional example of the design differences made for USA produced weapons; the Chrysler Corporation redesigned ten components to suit mass production techniques and this was claimed to have saved some 7,500,000 pounds (3,402,000 kg) of material and 1,896,750 man hours during a year's production, as well as freeing up 30 machine tools for the production of other components.

One firm rule adopted early in the redesign process was that any new Allied munition for these weapons needed to be completely interchangeable with existing designs. This allowed ammunition produced by any American or British ordnance manufacturer to be used with any weapon produced by either country, thus greatly simplifying the logistics problems of a world-wide war. In accordance with this rule, the USA originally adopted the British fuze design with the understanding that both the US Army and Navy "would be free to substitute components of proven reliability which would speed production." The fuze designed and produced in Britain was adopted as an interim measure by the USA, but this was considered to be of an unsafe design and unsuitable for mass production techniques. Fortunately, this fuze was almost immediately replaced by one designed by R.L. Graumann of the Naval Ordnance Laboratory. This fuze was simple in design and "ideally suited to mass production." Designated as the Mark 27, this new fuze was found to be 99.9 percent efficient in ballistic acceptance tests, a record not equaled by any other fuze of the time. Both the US Army and the British adopted this fuze for their own production lines. The USN estimated that the adoption of the Mark 27 saved some $250,000,000 during the war. Overall, the USA spent over $700,000,000 on 40 mm ammunition.

The first USN pilot twin was completed in January 1942 and the first quad in April 1942. The first shipboard quad installation was on the gunnery-training ship (ex-battleship) USS Wyoming (AG-17) on 22 June 1942, and the first twin installation was on the destroyer USS Coghlan (DD-606) on 1 July 1942. The USA started a massive production program for these weapons and a monthly production rate of 1,600 Army guns and 135 Navy twin-barrel guns was achieved by December 1942. Production continued to ramp up in the following year, so much so that the Army found that they had more guns than they could field and production of air-cooled single guns fell from a peak of 13,485 in 1943 to 1,500 guns in 1944 and then halted with no guns for the Army being produced during the last year of the war.

However, the needs of equipping Navy ships were not fully met until well into 1944. By that time, the pre-war 1.1" gun had been almost totally replaced by Bofors guns. During 1944, 6,644 single mountings and approximately 3,650 twin and 750 quad mountings were produced with production still ramping up with at least 796 single, 3,020 twin and 800 quad mountings being manufactured in 1945 until the end of the war halted production. To illustrate how many of these weapons were required by the USN, note that out of the more than 400 destroyers built for the USN between 1934 and 1946, only the four destroyers of the pre-war Gridley class (DD-380) and those destroyers sunk early in the war did not receive at least some Bofors guns.

Part of this increasing demand was the late war program of replacing 20 mm Oerlikon guns with the Bofors 40 mm guns, as the smaller weapon was found to be ineffective against Japanese Kamikazes. However, by early 1945, even the Bofors was determined to be inadequate against suicide attacks. As a result, a crash program was started late in the war to develop a new rapid fire 3"/50 (7.62 cm) gun to replace the Bofors. It should be noted here that although the Bofors gun was probably the best anti-aircraft automatic cannon of World War II, the USN considered it to be a front-line weapon for only six years. It did remain in service in the USN until the 1970s on auxiliary and non-modernized ships, primarily because the 3"/50 RF (7.62 cm) replacement program was never fully funded. As late as 1977, the US Navy still had twenty-four Mark 3 Mod 0 air-cooled single guns and carriages on the active fleet list. In 1969, air-cooled Bofors guns provided from Naval depots were used to arm Air Force AC-130 "Spectre" gunships for use in Vietnam.

As the USN decommissioned and transferred ships to other nations following the end of World War II, ex-US Bofors guns saw service under many flags with some still in active use as late as 2013.

Early versions of the Bofors twin mounting used friction-coupled drives, which quickly wore out on naval ships due to salt contamination. Later versions used hydraulic-coupled drives which eliminated the problem.

The development of the Mark 51 director system gave the USA weapons greatly improved accuracy. For example, half of all Japanese aircraft shot down between 1 October 1944 and 1 February 1945 were credited to the Bofors/Mark 51 combination. See the essay on the Mark 51 director on the History and Technology section for additional information.

The USN 40 mm/56.3 Mark 1 and 40 mm/56.3 Mark 2 Bofors guns were both water-cooled and were used for all twin and quad mountings. The Mark 1 was a left-hand weapon while the Mark 2 was a right-hand weapon. Except for the barrel assemblies, the components were not interchangeable. These weapons could be fired in single-shot or automatic mode via a selector switch on the side of the slide.

The 40 mm/56.3 M1 was an air-cooled version originally produced for the US Army. The barrel assemblies for the M1 were interchangeable with those of British and Canadian produced air-cooled weapons. All USN single mountings used a modified version of the Army M1 gun.

German forces captured many Bofors guns during 1939 - 1941 and most of these were either sold back to Sweden in exchange for raw materials or placed into storage. However, after the defeat of France in 1940, Germany had so many captured guns that it was decided to adopt them into the armed forces and they were then designated as the 4 cm/56 Flak 28.

Earlier in 1940, the German invasion of Norway had halted production at the Waffenfabrik Kongsberg facility just as their first four Bofors guns were nearing completion. Waffenfabrik Kongsberg had obtained a license from Bofors in July 1937 but took years to set up a production line. The production line sat idle for a few months following the surrender but then restarted under German control. Repair of damaged captured guns and production of new ammunition was also started at this facility. Most of the new guns were sent to the Kriegsmarine where they were used on the cruisers Admiral Hipper and Prinz Eugen and on Schnellbootes. From 1941 onwards the Hungarian firm of MAVAG, which had also obtained a pre-war license from Bofors, produced Bofors guns for Germany as well as their own armed forces. By July 1941 the Kriegsmarine had 247 guns in service and by March 1942 the Luftwaffe had 615 guns in service.

Besides purchasing weapons directly from Bofors, governments and manufacturing firms in Austria, Belgium, Czechoslovakia, Denmark, Finland, France, Greece, Hungary, The Netherlands, Norway, Poland, Portugal and the United Kingdom all obtained manufacturing licenses during the 1930s, but not all of these countries had started actual production before the start of World War II. Some nations started producing guns after 1939, but Czechoslovakia, Denmark, France, Portugal and Greece never did. Japan developed her own derivative of the Bofors gun from captured British weapons, as detailed below.

Poland was one of the first to start manufacturing and actually exported 168 guns to the United Kingdom, France, Romania and The Netherlands. Some of those intended for the United Kingdom and The Netherlands were stranded in Sweden when that country invoked their neutrality laws in January 1939, even though war was not declared until September. These were eventually taken over by the Swedish armed forces. Poland also supplied 24 guns and 50,000 rounds of ammunition in 1938 to the Spanish Republicans which were eventually captured by the Nationalists and were used as coastal defense with fourteen still in service as late as 1958.

A Note on Sources: "The Bofors Gun" by Gander says that L/43 guns were sent to Poland and used on the Polish destroyer Blyskawica and other ships but a long Email discussion and examination of Polish pre-war photographs supplied by Maciej Tomaszewski has convinced me that Blyskawica was actually armed with L/60 guns. At this time, he has not found any evidence that any of the Bofors guns sent to Poland were L/43 type.

Additional information on guns produced and used by many nations may be found in the Mount / Turret Data section at the bottom of this page.

• ^Values for HE Shell from BuOrd OP 1188 page 2 and "Naval Weapons of World War Two" page 147. The muzzle velocity shown is the expected average MV over the life of the gun barrel.
• ^The maximum effective range with the Mark 51 control system was about 1,500 yards (1,370 m). With the post-war, radar equipped Mark 56 and Mark 63 systems the effective range was about 2,500 yards 2,290 m).
• AP projectiles had a blunter nose and as a result their maximum range was about 9,620 yards (8,800 m).
• USA produced HE-SD ammunition was set to self-detonate at 4,000 - 5,000 yards (3,700 - 4,570 m) so as to minimize problems due to "friendly fire." HE and AP rounds that did not self-destruct were also manufactured.

Prior to the start of World War II Bofors had produced 182 Land-Service guns for the Swedish Army and 295 for export. Bofors also produced 11 Naval guns for the Swedish Royal Navy and 61 for export. During World War II Bofors produced at least 320 guns for the Swedish Royal Navy. Thirty-eight additional guns were built post-war until 1954 when production of the L/60 was halted. Guns were of both air-cooled and water-cooled types and at least fifteen different single, twin and submarine mounting types were in Swedish service. Some of the twin water-cooled mounting types had integral 1.25 m or 2.0 m Hazemeyer rangefinders.

• Serious British interest in this weapon was first shown by the Army in 1933 and was followed by an order for 100 guns in 1937. Later, a manufacturing license was purchased from Bofors. The British version is officially listed as 56.3 calibers long. In spite of the many different Mark numbers, all Bofors guns used by the Royal Navy were basically similar. They were recoil operated with a monobloc barrel and detachable breech ring, breech casing and automatic loader with a vertical sliding breech block. The air-cooled Marks I, I* and III differed in details of the automatic loader and were primarily derivatives of Land Service (LS - Army) guns. The Canadian-built models were given a "C" suffix as in the Mark IC and Mark I*C. The Australian-built Mark I* was identical to the British-built gun of that designation. The water-cooled Marks IV, VIII, IX, X, XI and post-war NI and NI/I all had water jackets with circulating pumps and differed only in regards to the mounting they were to be used on. Single shots could be fired in all but the Mark VIII, IX, X and probably the NI and NI/I guns. Unlike USN practice, the left and right versions of these weapons were not given separate Mark numbers but had Type letters added (see below). This large number of variations of British Bofors guns compared to the three produced for the USN would seem to show once again the lack of interest by the Royal Navy in weapon standardization during the World War II period. The British considered the Bofors to be at least twice as effective as their own 2-pdr against torpedo bombers, but not much better than that weapon against kamikazes. This weapon was not used in large numbers on British ships with the largest totals being 14 on the escort carrier HMS Trouncer and 12 on the fleet carrier HMS Indomitable and the light fleet carrier HMS Ocean. Details on the guns:
  

  Marks I, II and III: Air-cooled, LS guns, some of which were converted to naval use. See Note 3e below.

  Mark IV and IV/I: Water-cooled. Designed for use on the twin-gun Mark IV Hazemeyer mountings. Produced in left and right versions, with Type D being the left gun and Type E the right gun. Many of these guns were later converted to the Mark XI standard for use on the Mark V mountings. A total of 484 guns were produced with 442 in service by 1945. See Note 4e below.

  Mark V: Project abandoned

  Mark VI: Army gun issued to Crusader AA tanks.

  Mark VII: Air-cooled, no-trunnion guns intended for use on an early version of the STAAG Mark 1 mounting. 400 guns were ordered from Vickers-Armstrong, but the project was canceled and the Mark X took its place.

  Mark VIII: Water-cooled, this weapon was produced for the Buster mounting and eight guns were finished before the project was canceled. Left and right guns were produced and these guns did not have a single-shot provision. See Note 12e below.

  Mark IX: Air-cooled guns for the for the Mark VI six-gun mounting. Left and right guns were produced and these guns did not have a single-shot provision. See Note 6e below.

  Mark X: Water-cooled guns for the STAAG Mark 2 mounting. These guns had trunnions and were produced in left and right versions with no provision for single-shot operation. See Note 11e below.

  Mark XI: Water-cooled guns for the Mark V mounting. Many of these were converted from the Mark IV and Mark IV/I. Total of 342 in service by 1945.

• The USA provided 393 each of their Mark 1 (left-hand) and Mark 2 (right-hand) guns to the Royal Navy. These were used in British RP Mark I (Twin) and RP Mark II (Quad) mountings. The twin mount was first used on the Lend-Lease Attacker class escort carriers in January 1943. The quad mount was first installed on HMS Phoebe in June 1943. The USA also supplied air-cooled guns which were used mainly on LSTs.
• The Mark III series of hand-operated single mountings were the Army design adapted for Naval use and were widely used, with some 500 in service by the end of World War II. Usually designated as LS Mark III (Land Service). The following mountings do not appear to have entered naval service, although they do appear on naval gun lists: Mark III* (hand operated with gyro sights for layer and trainer), RPLS Mark III (Remote Power Land Service) and Toadstool (joystick controlled power operation using Army components). Elevation limits for the Mark III were -5 to +90 degrees. As of June 1942, 314 Mark III mountings were in service, of which 301 were on DEMS. By May 1945, there were about 1,392 in service with 568 on DEMS. Those Army mountings modified at least somewhat for naval service were designated as Mark III CN and there were 500 of these in service at the end of the war, with 291 on DEMS. Mounting weight for the LS Mark III including the gun was 1.2 tons (1.22 mt).
• The Mark IV twin mounting was derived from the Hazemeyer triaxial mounting which had its origins in the 1940 arrival in Britain of the Dutch minelayer Willem van der Zaan. The Mark IV was a self-contained twin mounting that had its own rangefinder, radar and analog computer on the mount. This mounting used Mark IV water-cooled guns and utilized a track and pinion system for elevating and training which was powered via a Ward-Leonard system for automatic target tracking. The Mark IV was probably too advanced for its day and proved to be somewhat delicate for use on destroyers and sloops. The later STAAG and Buster designs were more robust, but very much heavier. According to service notes, the Mark IV was apparently used more often in manual mode than in power mode. Elevation was -10 to +90 degrees with cross-level of +/- 14 degrees with control cutting out at +/- 12 degrees. Maximum elevating speed was 25 degrees per second with the manually controlled joystick, but training and elevation via automatic control was limited to little more than 10 degrees per second. Weight was 7.05 tons (7.16 mt). The later Mark IV* mounting had a more robust radar attachment and differed in details of the controls and gyros. The following description taken from "Destroyer Weapons of World War 2" is of interest:

  'The 7-ton "Haslemere," as it was generally known, was a brilliant concept, but unfortunately it needed more advanced technology than then existed. It cannot claim to have been the most popular of weapons but at least it provided a little light relief on occasions. When stationary in the "power-off" mode during maintenance, a combination of training, depression and cross-roll made it look for all the world as though it was about to fall off its gundeck. Observations like "I see the Haslemere is ill again" were common.'

• The design of the Mark V twin was based upon the USN Mark 1 twin mounting adapted to use British components including some components of 2-pdr mountings and RPC gear from the 4.5" (11.4 cm) Mark V. Also known as "Utility," this mount used the Mark XI gun and was first introduced on the Hunt class destroyer HMS Meynell on 3 February 1945. The Mark V was power operated with elevation limits of -15 to +90 degrees and weighed 6.4 tons (6.5 mt) with 12 four-round clips carried on mount. Post-war, the minimum elevation was modified to -14 degrees. The Mark V proved quite popular in service and was retained in service long after the more sophisticated STAAG was retired. Its largest drawback was the lack of a blind fire capability. The RP50 Mark V had a maximum training speed of 35 degrees per second and elevating speed of 28 degrees per second. The RP50 Mark VC (Canadian) had a maximum elevation speed of 35 degrees per second.
• The Mark VI was a sextuple mount using the air-cooled Mark IX gun and used a 36-round ammunition tray for each gun rather than the usual four-round clips. Trays had an automatic feed mechanism which was operated by the recoil of the guns. Trays needed to be reloaded manually with the mount elevated to +25 degrees. Left and right versions of the guns were produced and the guns did not have a single-shot capability. Training and elevating speeds of the RP50 Mark VI mounting were both 30 degrees per second. This mount did not enter service until after the war. Weight was 21.24 tons (21.58 mt). This was a large weapons platform intended to be fitted primarily to battleships and fleet carriers with three hundred-twenty four being completed. The first battleship fitted was HMS Vanguard and she eventually carried a total of ten mountings although this number was rapidly reduced during her career. Aircraft carriers HMS Eagle and HMS Ark Royal carried eight. On some ships and some mountings, the lack of working space meant that the guns could only be reloaded at certain fixed bearings while on other ships the reloading could occur with the guns traversing at an automatically slowed rate.
• The Mark VII was an adapted Army single mount with a weight of 1.40 tons (1.42 mt). Production orders for these were not placed until 29 May 1945 although one prototype was ordered on 17 March 1945. Similar in design to the "Boffin" mounting, described below, but able to elevate to +90 degrees and could train continuously as it used a slip ring for electrical power.
• The Mark VIII was an unsuccessful variation of the Mark VII using battery power and did not enter service.
• The Mark IX was an upgraded Mark VII mount with electric drive. The Mark IX mount used the Mark NI gun and had six ready-use clips on mount. Mark IX was used successfully during the Falklands War, with guns on HMS Fearless and HMS Intrepid shooting down an Argentine A-4B Skyhawk on 27 May 1982, probably the last aerial victory scored by a 40 mm/60. This success stimulated renewed interest in automatic weapons in the Royal Navy, eventually leading to the purchase of newer 20 mm and 30 mm weapons.
• The Boffin mounting was a twin 20 mm Oerlikon Mark V or Mark VC mounting modified to take a single Bofors 40 mm gun. Elevation was -10 to about +70 degrees. These used an oil hydraulic system and were fitted with a gyro gunsight. Elevation was restricted compared to other Bofors mountings as the position of the elevating trunnion axis was lower, due to the smaller nature of the Oerlikon guns. Some of these were still being used by the Canadian Navy in 1990 during the Persian Gulf war.
• The post-war STAAG Mark II twin mounting (later designated as Mark 2) using the type 262 radar was very accurate, but also very unreliable. Part of this unreliability appears to have been a result of the decision to mount the radar equipment directly to the gun mount, thus exposing it to a high level of vibration. At 17.5 tons (17.8 mt), it was also quite heavy for only a twin weapon. Used the Mark X gun. The hydraulically powered Mark II was called a "pseudo-triaxial" mounting and was unusual in that the third axis was a lateral deflection movement instead of being cross-roll. Training and elevating speeds were both 35 degrees per second. STAAG was found to be overly complicated and difficult to maintain and did not enjoy a long service life as a result. As noted, the type 262 radar was very accurate and there are stories of guns in action against towed targets where after shredding the sleeve the radar would track up the tow wire and the tow plane would escape only by cutting the wire. Nylon rope was eventually substituted for the wire as a remedy. The Mark I was the prototype single mounting. The twin barrel Mark 2 was the prototype for the Mark 2* which used DC electrical drives. The Mark 3 used AC drives. The Mark 2** had no radar in the mounting and was used for training.
• The BUSTER (Bofors Universal Stabilized Tachymetric Electric Radar) twin was another World War II attempt at a self-contained mounting, but it was a very large platform and at approximately 20 tons (20.3 mt) this weighed far too much for only a twin arrangement and the project was canceled. Used the Mark VIII gun. See sketch below.
• Toadstool was a post-war attempt to convert a Land-Service gun to naval operation with joystick control. Extensively tested but did not enter service.

• ^1.11.2Mark 3 single mounts used air-cooled Navy M1 guns, which were modified versions of the US Army M1 Bofors gun. Three versions of the Mark 3 were used on surface ships, the Mod 0, Mod 4 and Mod 9, while submarines used the "wet mount" Mod 5 and Mod 6. Mod 0 was the basic Army mount and weighed 2,440 lbs. (1,107 kg) including gun and sighting mechanisms. The Mod 0 lacked power drives and so was manually trained and elevated. Crew for the Mod 0 was usually five to six men. Mod 4 added 1 hp power drives for training and elevation and had the same crew size as the Mod 0. These power drives differed from those made for the Army as the Naval mounts needed to be more powerful in order to compensate for the movements of the ship such as roll and pitch. Submarine Mods 5 and 6 were manually worked mountings. Most shipboard installations of manually worked single mounts had safety rails around them to keep the gun crews from accidentally firing into the ship. The Mark 3 Mod 9 was a much later design using rebuilt M1 guns. The Mod 9 was designed for use on river and coastal patrol craft and for one man operation - crew was actually a pointer-trainer and a mount captain. This mount used integral train and elevation power drives and was stabilized. Weight increased to 4,200 lbs. (1,905 kg). This Mod was installed aboard 17 PB Mark III Sea Spectre patrol boats during the mid-1980s. Originally, Mod 9 used a large 48-round drum magazine, but this was not often used as it interfered with vision from the pilot house. This ammunition drum looked something like the ones used on 20 mm Oerlikons guns but, of course, much larger. During the 1980s "Tanker War" in the Persian Gulf, it was reported that the drum did not feed reliably, so it was removed and the crews went back to manual loading.
• ^Recoil needed to be at least 7.2 inches (18.3 cm) to insure automatic operation of the weapon. Normal recoil varied from 7.7 to 7.9 inches (19.5 to 20 cm) with the maximum recoil being about 8.3 inches (21.1 cm).
• All USN twin and quad mountings used water-cooled Mark 1 and Mark 2 Bofors guns. Any mod of the Mark 1 or Mark 2 Bofors guns could be used in any mod of USN twin or quad mountings; these USN guns were designed to be completely interchangeable in that regards. Twin mounts consisted of a left-hand gun (Mark 1) and a right-hand gun (Mark 2) joined together. The gun axes were 9.568 in (24.3 cm) apart. Elevation and training motors were 3 or 5 hp. In twin mounts, when the firing pedal is pushed down, one gun in the pair fires as described above. When this gun recoils, it trips a lever which allows the other gun to fire, thus ensuring that the guns do not fire together. For twin and quad mounts, the crew consisted of a Mount Captain, Pointer, Trainer, a 1st Loader for each gun and then however many ammunition passers it took to get back to the ammunition supply point. In addition to the guncrew, there was normally a Mark 51 (or later) Director crew consisting of a Pointer and a Range Setter.
• Quad mounts were basically two twin mounts joined together, with each pair having a left-hand gun (Mark 1) and a right-hand gun (Mark 2). The gun pairs axes were 60.0 in (1.524 m) apart. Elevation motors were 5 hp and training motors were 5 or 7.5 hp. Hydraulic drive gear was used on most twin mounts. Although both pairs of guns elevated together, in some Mods the pairs could be uncoupled in case of damage. As noted above, twin mounts fired one gun at a time. In quad mounts the same delay gear was used, which meant that one gun in each pair fired at the same time, thus preventing unbalanced forces and reducing dispersion.
• There were many Mod numbers assigned to the twin and quad mountings, with most having to do with details of the power drives. An asterisk (*) indicated that the mount included a radar antenna and was used with the Mark 63 director.
• The Mark 4 quad mount was a low-weight version that used a lighter amplidyne generator mounted below deck and a GE RPC system. This mounting had much faster training and elevation speeds than earlier mounts, but only 100 had been delivered by the end of the war.
• US ships carried large quantities of this weapon with USS Saratoga (CV-3) probably having the most with 23 quad mounts and two twin mounts for a total of 96 guns. Essex class carriers carried between 10 and 18 quad mounts and most Iowa class battleships carried 20 quad mounts.
• A Note on Sources: According to "U.S. Navy Bureau of Ordnance in World War II," the USN built more than 2,300 quad mounts, just under 10,000 twin mounts and more than 10,000 single mounts during the war. This mass production gradually reduced the cost, with quad mounts dropping from $86,000 down to $67,250 and twin mounts dropping from $62,300 down to $43,640. "The Bofors Gun" quotes different quantities for these mountings, but does not note the source for the numbers stated.
• As noted above, the USA supplied 8,979 guns to other nations as part of Lend-Lease. This is broken down as follows:

      United Kingdom and Commonwealth: 2,834
      China: 80
      Free French Forces: 448
      USSR: 5,511
      American Republics: 4
      Others: 2

As far as is known, only single mountings were ever used on warships and only HE and HE tracer was issued. The Kriegsmarine received a total of 578 guns for use on ships with an additional 247 guns delivered as coast-defense weapons. Naval Flak 28 mountings were set on a rotating ring attached to a round base plate that was fixed to the deck. Elevation was -6 / +90 degrees and training was 360 degrees.

The Type 5 (Model 1945) originated from the capture at Singapore in 1942 of 24 British Bofors single air-cooled guns in working condition and another 56 made serviceable after repairs. One of these was used to produce a Japanese copy and this underwent prototype firing trials in 1943 at the Torigasaki range of the Yokosuka Naval Arsenal. Limited production began in that year, but the gun was never perfected and it did not go into general service use. However, some 5 to 7 weapons a month were being produced in late 1944, apparently for service evaluation purposes. The main alteration from the British Bofors design was to increase the bore length to 94.49 in (2.400 m) - 60 calibers - and to add Rhienmetall-style flash suppressors, which proved unsatisfactory. Production was at the Yokosuka Naval Arsenal and at the Hitachi Manufacturing Company. Used only in manually-worked single mountings, which had an elevation range of -10 / +95 degrees and a weight of 1,870 lbs. (850 kg). Recoil was 8.5" (21.6 cm). Major problems found by the USN after the surrender were that poor manufacturing practices and lack of quality control caused improper seating of rounds and jamming of parts and that the star wheels and extractors were frequently mismated.

Nomenclature Note: Although the Japanese designation is normally described as the Type 5 (Model 1945), "US Naval Technical Mission to Japan report O-47(N)-2" at one point (page 16) describes the Mechanism designation as "Type 5 (1943)," which cannot be correct as the numbers do not match. This error may imply that the designation would actually be "Type 3" (Model 1943) which would seem to be more in keeping with the actual Japanese design date of 1943. However, it should be noted that the Japanese Model number system, normally based upon the year the breech design was started, became very chaotic towards the end of World War II, with new weapons having Type years that had no relationship to the actual year that the breech design was started. So, this weapon may indeed have been designated as Type 5.

Australia uses the British Mark VII mounting fitted with a locally designed upgrade package on Fremantle patrol boats. Elevation was -5 / +90 degrees and train was 360 degrees. Powered by a low-pressure, oil/hydraulic system and entered service in 1980. Training speed of 20 degrees per second and elevation speed of 40 degrees per second. Designated as 40/60 AN. A small number of these mountings were sold to Thailand.

The Republic of Korea has developed a modification kit for the USA Mark 1 twin mount which adds a stabilization system both to the gun mount and to the Mark 51 FCS. This kit improves their effectiveness against surface targets. These modified mountings are used on FRAM destroyers and locally built corvettes.

Hazemeyer was a Dutch subsidiary of Siemens Halske. Prior to World War II, this firm developed a very advanced triaxial mounting together with a tachymetric control system. As noted above, upon the Dutch defeat in 1940, this mounting was brought to Britain where it was immediately copied and introduced into production.