Updated 06 February 2016
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. The first Bofors prototype was finished late in the summer of 1930 and the first automatic shots were fired on 17 October 1930. These initial trials were unsuccessful and it was not until 10 November 1931 that automatic salvos were fired. Official trials for the Swedish Navy took place on 21 March 1932. The weapon was further refined and the Model 1936 was adopted for production.
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 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 air-cooled twin version from Bofors in early 1940. This arrived in New York from Sweden on 28 August 1940. During the same month, the Dutch escort vessel van Kinsbergen demonstrated these weapons to USN observers in a test off Trinidad. BuOrd formally obtained Swedish licenses in June 1941, although some manufacturing actually started prior to that time. Terms of the license included $500,000 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 USN 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 ships of the US Navy. 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 production engineer supposedly 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 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.
For ammunition, 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." The new fuze, designated as the Mark 27, 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.
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.
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 by December 1942 a monthly production rate of 1,600 Army guns and 135 Navy twin-barrel guns was achieved. A total of about 42,895 guns were built by the end of the war, distributed as 23,897 for the Army, 10,019 for the Navy and Coast Guard, and 8,979 for export to other nations. Even so, the demand was not fully met until well into 1944. By that time, the pre-war 1.1" gun had been almost totally replaced by Bofors guns. To illustrate how many of these weapons were produced by the USA, 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.
Late in World War II, the USN started replacing 20 mm Oerlikon guns with the Bofors 40 mm guns, as the smaller weapon was found to be ineffective against Japanese Kamikazes. However, even the Bofors was determined to be inadequate against suicide attacks by early 1945, and as a result Bofors guns were in turn replaced during the late 1940s and 1950s with the new rapid fire 3"/50 (7.62 cm) designs. It should be noted that although the Bofors gun was probably the best anti-aircraft heavy machine gun 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.
Early versions of the 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 article on the Mark 51 director on the Technical Board for additional information.
The USN Mark 1 and 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 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.
These guns are recoil operated and use a monobloc barrel with a detachable breech ring, breech casing and automatic loader. Breech block is a vertical sliding type. Although often listed as being 60 calibers long, all of these guns except for the Japanese version were actually 56.25 calibers in length.
Additional information on guns produced by all nations may be found in the "Mount / Turret Data" section at the bottom of this page.
USA 40 mm/56 Quad Mount on USS Hornet CV-12
(see Note 1)
|Sweden: Bofors 40 mm/60 (1.57")
Germany: 4 cm/56 (1.57") Flak 28
Japan: 4 cm/60 (1.57") Type 5 (Model 1945)
UK: 40 mm/56.3 (1.57") QF Marks I, III, IV, VIII, IX, X, XI, NI and NI/I
USA: 40 mm/56 (1.57") Mark 1, Mark 2 and M1
|Ship Class Used On||Almost all major US and British warships
of World War II
German Cruisers and S-Boats
|Date Of Design||Sweden: 1936
|Date In Service||Sweden: N/A
Japan: Not in service
|Gun Weight||Germany: N/A
Japan: 1,018 lbs. (462 kg) [air-cooled]
UK: 1,120 to 1,163 lbs. (508 to 528 kg) [depending upon Mark] [water-cooled]
USA Mark 1 and Mark 2: about 1,150 lbs. (522 kg) [water-cooled]
USA M1: 1,036.5 lbs. (470 kg) [air-cooled]
|Gun Length oa||Germany: N/A
UK: 145.3 to 145.5 in (3.691 to 3.696 m) [depending upon Mark]
USA: 148.8 in (3.780 m)
(see Note 9)
|Germany: 88.578 in (2.250 m)
Japan: 94.5 in (2.400 m)
UK: 88.578 in (2.250 m)
USA: 88.6 in (2.250 m)
|Rifling Length||Germany: 76.06 in (1.932 m)
Japan: 78.8 in (2.000 m)
UK: 75.85 in (1.927 m)
USA: 75.85 in (1.927 m)
(see Note 10)
Japan: (16) 0.0098 in deep (0.25 mm)
UK: (16) 0.0236 in deep x 0.220 (0.60 x 5.59 mm)
USA Mark 1 and Mark 2: (16) 0.0236 in deep x 0.220 (0.60 x 5.59 mm) [water-cooled]
USA M1: (16) 0.0225 in deep x 0.220 (0.057 x 5.59 mm) [air-cooled]
UK: 0.0892 in (2.66 mm)
USA: 0.0892 in (2.66 mm)
Japan: Uniform RH 1 in 30
UK: Increasing RH 1 in 45 to 1 in 30
USA: Increasing RH 1 in 45 to 1 in 30
|Chamber Volume||UK: 28.661 in3 (0.470
USA: 28.3 in3 (0.464 dm3)
|Rate Of Fire
(see Notes 2 and 6)
|120 rounds per minute per barrel nominal
140 to 160 rounds per minute when firing horizontal (gravity assist)
1) The official designation for British guns was Ordnance, Quick Firing (usually abbreviated as OQF) 40 mm Mark (whatever). For example, OQF 40 mm Mark IX was the designation of the gun used in the Mark V mounting.
2) A modification kit was produced around 1970 which increased the rate of fire to 180 rounds per minute and the magazine capacity to 20 rounds by using a banana feeder fed by standard four-round clips.
3) USN Mark 1 guns fed from the left while the Mark 2 guns fed from the right. Manually loaded M1 guns fed from the left.
4) The standard automatic loader holds two four-round clips. When the first four-round clip is inserted into the feeder, the clip itself is stripped off and falls out onto the deck (the clip chute is a cut out just below the loader; left side for a left gun and right side for a right gun). The second clip is then dropped into the loader and pushed down so that it forces a round through the loader star wheels and onto the rammer tray. This first round only had to be manually pushed through when the gun was initially loaded, the loader will automatically feed rounds from new clips. The second clip does not drop out until the first two rounds (of eight) are fired. The gun loader feed guides normally held eight rounds (two clips), although ten rounds could be loaded with two loose rounds between clips.
5) To get the gun ready to fire the first round, a cocking lever is used to manually move the rammer to the cocked position, rotate the star wheels a quarter turn which drops a round onto the rammer tray and lower the breech block. Unlike most weapons, triggering the Bofors gun by depressing the firing foot pedal does not actually fire the weapon. Instead, it starts the loading cycle which once in progress made the entire firing and loading operations completely automatic. At the start of this cycle, the spring-powered rammer is released, pushing the round on the rammer tray into the breech which automatically closes when the cartridge case rim strikes the actuator. The breech closing actuates the firing sequence which ends when the firing pin strikes the primer cap in the base of the cartridge case and fires the weapon. Recoil then opens the breech block, pushes the rammer back into the cocked position and the extractor arms pull the cartridge case out and back over the rammer tray and down a semi-circular chute that guided the spent case out under the gun barrel. As the breech block moves back during recoil, it causes the star wheels in the autoloader to make a quarter turn which drops the next round onto the rammer tray. In automatic mode, this cycle will repeat as long as the firing pedal is depressed and ammunition is in the autoloader. Mark 1 and Mark 2 guns had a loader interlock which automatically halted firing when there were only two rounds remaining in the loader, one on the rammer tray and one in the star wheels. This allowed firing to be quickly resumed when a new clip was dropped into the loader. Unlike the Mark 1 and Mark 2 guns, the M1 guns had a switch on the back of the loader at the center, bottom rear. This switch disabled the interlock so that the M1 could fire all eight rounds in the feed clips or it could be set to stop the gun when only two rounds were remaining, similar to the Mark 1 and Mark 2. If all rounds were fired, then the first loader would have to start from the beginning to drop in two clips and push on the top so that the bottom round would rotate through the loader star wheels and drop onto the rammer tray. The gun would then resume firing when the pointer pushed his foot pedal.
6) It was up to the second loader to properly orientate each clip when he handed it to the first loader so that all the first loader had to do was drop it into the loader. Since the Bofors gun cycled between 120 to 160 rounds per minute (one clip every 1.5 to 2 seconds), the first loader had to do a quick "pick up, turn, and drop" action in order to keep up with it. If the clip was not orientated properly during the hand-over, then the first loader would have to juggle it in order to position it properly before it could be dropped into the loader. Being too slow would interrupt the firing cycle - hence the reason for the loader interlock. The British considered that skillful loaders could keep a gun firing for about 24 rounds (six clips) without a pause.
7) Perhaps unusually for US guns, the bores of these weapons were not chromium plated.
8) Ammunition was percussion fired in all models.
9) In "US Naval Technical Mission to Japan report O-47(N)-2" it is stated that the Japanese in copying a captured British air-cooled gun 'increase[d the] barrel length from 2160 mm (85.1") to 2400 mm (94.57")'. This dimension is usually taken as being the overall barrel length in most references (see for example "Naval Weapons of World War Two"). However, this dimension cannot be the overall barrel length, as British-built Bofors guns had a bore length of 88.578 in (2.250 m) - 56.3 calibers, while their overall length was about 145.3 in (3.691m). As the rifling length given in O-47(N)-2 for the Type 5 is significantly longer than that for the British Bofors, I believe that the dimensions given in O-47(N)-2 for barrel lengths must actually be for the bore lengths and use this assumption in constructing this data page.
10) "German Naval Guns: 1939 - 1945" by Miroslaw Skwiot says that the rifling in German Bofors guns consisted of "sixteen grooves, each 41.2 mm wide." These figures are mathematically impossible. It is possible that the author (or the translators) made an order of magnitude error and that the actual width was 4.12 mm wide, but this dimension would still be significantly narrower than the grooves in UK and US guns.
|Projectile Types and Weights
(see Notes 2 and 3)
AB 40 mm L/60 HE-T - 2.05 lbs. (0.93 kg)
|Weight of Complete Round||Bofors - 4.63 lbs. (2.1 kg)
UK - 4.88 lbs. (2.21 kg)
USA - 4.75 lbs. (2.15 kg)
Germany - 4.6 lbs. (2.1 kg)
Others - N/A
|Bursting Charge||Bofors Model 1934
HE-T - about 0.150 lbs. (0.068 kg) TNT
Bofors (modern day)
USA (World War II)
Britain (World War II)
|Projectile Length||USA HE Mark 1 and Mark 2 - 7.25 in (18.4
USA AP M81A1 - 6.19 in (15.7 cm)
Germany - 7.2 in (18.4 cm)
Others - N/A
|Complete Round Length||Bofors - 17.60 in (44.7 cm)
USA HE - 17.62 in (44.75 cm)
Britain HE - 17.65 in (44.83 cm)
Britain AP and SAP - 17.58 in (44.65 cm)
Germany - 17.8 in (44.7 cm)
Others - N/A
|USN Cartridge Case Type, Size and Empty Weight||Mark 1 - Brass, 40 x 311R, 1.93 lbs. (0.88
Mark 2 - Brass, 40 x 311R, 1.89 lbs. (0.86 kg)
Mark 3 - Steel, 40 x 311R, 1.53 lbs. (0.69 kg)
|Propellant Charge||Bofors - about 0.661 lbs. (0.300 kg)
USA - 0.694 lbs. (0.314 kg) NC 025
USA - 0.661 lbs. (0.300 kg) SPDN
UK - 0.547 lbs. (0.248 kg) Cordite W
UK - 0.719 lbs. (0.326 kg) FNH/PO22
Germany - 0.668 lbs. (0.303 kg) Str PC/38N
Japan - 0.617 to 0.661 lbs. (0.280 to 0.300 kg)
(see Note 8)
|Bofors Model 1934: 2,798 fps (850
USA HE w/NC 025: 2,890 fps (881 mps)
UK HE: 2,800 fps (853 mps) with Cordite W
UK HE: 2,890 fps (881 mps) with FNH/PO22
Germany Flak 28: 2,801 fps (854 mps)
Japan Type 5: 2,953 fps (900 mps)
(see Note 8)
|Japan - 16.50 tons/in2 (2,600
UK - 19.68 tons/in2 (3,100 kg/cm2)
USA - 19.5 tons/in2 (3,070 kg/cm2)
Others - N/A
|Approximate Barrel Life||USA - 9,500 Rounds
UK - 10,000 Rounds
Germany - 10,000 Rounds
Japan - N/A
|Ammunition stowage per gun||Germany
Admiral Hipper and Prinz Eugen: 2,000 rounds
S-Boats: 500 rounds
1) Ammunition for most guns is held in four-round aluminum clips. The feed guides atop each gun can hold two clips at a time. Two loose rounds can be inserted between clips. A four-round clip weighs about 19.0 lbs. (8.6 kg).
2) The many Mods of USN ammunition were primarily bookkeeping designations used to indicate the manufacturer. USN AP had a windscreen. USN HE ammunition was issued in HE-T/SD, HE/SD, HE-I-T/SD and HE-I/SD forms. Plugged forms for training purposes were also manufactured. There was also a Mark 3 HE round that did not contain a tracer, but this was used only for a brief time during World War II and was replaced in 1945 by Dark (non-luminous tracer) and Dark Ignition (delayed ignition tracer) ammunition. Dark Tracer was issued only in HE/SD and HE-I-T/SD forms. Dark Ignition ammunition was issued only in HE-I-T-SD form.
3) By 1941 Britain was producing forty-one marks of HE and HE-T, nine marks of SAP, five marks of AP, twenty-four marks of TP and TP-T, six marks of proof ammunition, four marks of barrel-cleaning rounds, five marks of break-up ammunition for training and gun checking, and a single mark of paper shot ammunition. This plethora of types threatened to overwhelm the logistical system and a rationalization program reduced the numbers down to three HE and HE-T marks by 1945. Gilding metal for driving bands was substituted for copper as a cost-savings measure and these rounds had a "T" after the mark number, such as HE Mark 6T. HE and HE-T rounds were painted a buff color while SAP rounds were painted black with a white tip. AP rounds were painted black with a white tip and a white body ring. Tracer rounds added a red body ring.
4) USN tracer burned out at 5,000 yards (4,570 m) horizontal, 15,000 feet (2,740 m) vertical.
5) USN bourrelet diameter was 1.570 inches (39.88 mm).
6) German rounds were HE with tracer and self-destruct. Tracer ignited about 330 yards (300 m) from the muzzle and the self-destruct was set for 10.5 seconds.
7) Modern ammunition: Bofors introduced a 40 mm PFHE proximity round during the 1980s similar to those developed for their larger guns. Bofors claims a maximum effective bursting radius of 18 feet (5.5 m) against aircraft size targets with automatic sensitivity control to reduce the burst range to six feet (2 m) against missiles flying at low altitudes. Muzzle velocity is 2,820 fps (860 mps) and the total shell weight is 2.16 lbs. (0.98 kg) including 3.2 oz (90 gms) of octol explosive. Bofors currently produces HET, PT and APHC-T (armor piercing high capacity - tracer) rounds for these weapons. The APHC-T round is unusual in that it carries an armor-piercing slug within an aluminum body. Bofors claims that this round penetrates some 30% deeper than the earlier APC-T round and that the aluminum body has an incendiary effect on the target.
8) In "US Naval Technical Mission to Japan report O-47(N)-2" it is stated that the muzzle velocity of the Japanese weapon was 2,822 fps (860 mps) before the barrel was lengthened and the propellant charge increased. However, I must question if the working pressure given in this document - 16.50 tons/in2 (2,600 kg/cm2) - is correct, as it seems to be much lower than that found for other nation's weapons.
|AP||Black||Black||Black||Plug in tracer|
|HE-P||Green||Green||Green||Plug in base|
|HE/SD (DT)||Green||Black||Green||Dark Tracer|
|HE-I-P||Green||Red||Red||Plug in base|
|HE-I/SD (DT)||Green||Black||Red||Dark Tracer|
with Black Band
|White||Red||No SD function|
|BL&P||Red||Red||Red||Dummy Fuze and Plug|
|HE-I-T-SD (DI)||Green||White||Red||Dark Ignition|
1) The Mark and Mod, manufacturer's initials or symbol, and lot number are stamped around the body of the projectile.
2) Color photographs of some of these projectiles may be seen on the additional pictures page.
|Elevation||With 1.985 lbs. (0.900 kg)
HE Mark 2 Shell
|With 1.960 lbs. (0.889 kg)
AP M81A1 Shell
|Range @ 10 degrees||6,844 yards (6,258 m)||6,466 yards (5,913 m)|
|Range @ 15 degrees||8,227 yards (7,523 m)||7,580 yards (6,931 m)|
|Range @ 20 degrees||9,295 yards (8,499 m)||8,389 yards (7,671 m)|
|Range @ 25 degrees||10,103 yards (9,238 m)||8,959 yards (8,192 m)|
|Range @ 30 degrees||10,691 yards (9,776 m)||9,358 yards (8,557 m)|
|Range @ 35 degrees||11,057 yards (10,111 m)||9,568 yards (8,749 m)|
|Range @ 40 degrees||11,208 yards (10,249 m)||9,618 yards (8,795 m)|
|Range @ 45 degrees||11,133 yards (10,180 m)||9,492 yards (8,679 m)|
|AA Ceiling||22,299 feet (6,797 m)||
1) USA produced HE-SD ammunition was set to 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.
2) Time of flight for 1.985 lbs. (0.900
kg) HE shell with MV = 2,890 fps (881 mps)
Muzzle velocity of 2,800 fps (853 mps)
|With 1.97 lbs. (0.894 kg) HE Shell with no self-destruct|
|Range @ 45 degrees||10,750 yards (9,830 m)|
|AA Ceiling||23,500 feet (7,160 m)|
|Note: British rounds normally self-destructed at 3,000 - 3,500 yards (2,700 - 3,200 m), but this was increased to 7,000 yards (6,400 m) in some ammunition types.|
Muzzle velocity of 2,800 fps (853 mps)
|With 2.105 lbs. (0.955 kg) HE Shell|
|Range @ 45 degrees||about 10,500 yards (9,600 m)|
|Maximum AA Ceiling||22,970 feet (7,000 m)|
|Effective AA Ceiling||20,340 feet (6,200 m)|
|Note: German rounds self-destructed after 10.5 seconds.|
Muzzle velocity of 2,953 fps (900 mps)
|With 2.21 lbs. (1.005 kg) HE Shell|
|Range @ 50 degrees||10,900 yards (10,000 m)|
|AA Ceiling @ 90 degrees||26,250 feet (8,000 m)|
|Maximum effective range
|3,280 yards (3,000 m)|
|Note: The USN attributed this low effective range to "poor fuze design." It is unclear as to how the fuze design affected the effective range, but I would assume that this meant that the fuze was of poor aerodynamic shape. Self-destructing ammunition was not used.|
Muzzle velocity of 2,890 fps (881 mps)
|0 yards (0 m)||
|2,000 yards (1,829 m)||
|4,000 yards (3,658 m)||
|6,000 yards (5,486 m)||
|Note: This data is from "Battleships: United States Battleships 1935-1992" and is based upon the USN Empirical Armor Penetration formula. Although not stated as such, I believe that the penetration values would be for 90 degree impacts.|
(4445 - 5897 kg)
(10,524 - 10,796 kg)
(1,107 - 1,905 kg)
(10,340 - 11,137 kg)
|Recoil||8 - 9 in (20 - 22 cm)|
|Notes on Bofors-built Weapons
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 service. Some of the twin water-cooled mounting types had integral 1.25 m or 2.0 m Hazemeyer rangefinders.
|Notes on USA-built Weapons
1) Mark 3 single mounts used air-cooled 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. Submarine Mods 5 and 6 were manually worked mountings. Most single mount shipboard installations 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 drum looked something like the ones used on 20 mm Oerlikons but, of course, much larger. During the 1980s "Tanker War" in the Persian Gulf, it was also reported that the drum did not feed reliably, so it was removed and the crews went back to manual loading.
2) 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.
3) 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. As noted above, hydraulic drive gear was used on most units. Although both pairs of guns elevated together, in some Mods the pairs could be uncoupled in case of damage.
4) 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.
5) 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.
6) 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.
7) 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 costs during the war, with quad mounts dropping from $86,000 down to $67,250 and twin mounts dropping from $62,300 down to $43,640.
8) 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:
|Notes on British-built
Weapons and Mountings
1) 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 Austrailian-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. Details on
Marks I, II and III: Air-cooled, LS guns, some of which were converted to naval use. See Note 3 below.2) 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.
3) 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).
4) 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.'
6) 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.
7) 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.
8) The Mark VIII was an unsuccessful variation of the Mark VII using battery power and did not enter service.
9) 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 the 40 mm/56. 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.
10) 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.
11) 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.
12) The Buster (Bofors Universal Stabilized Tachymetric Electric Radar) twin was another World War II attempt at a self-contained mounting, but 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.
13) 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.
|Notes on Australian Weapons
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.
|Notes on South Korean Weapons
South 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.
|Notes on Netherlands Hazemeyer
Weapons and Mountings
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.
|Notes on German Weapons
Guns intended for naval use were manufactured at the Norwegian Kongsberg Arsenal. The Arsenal started license production of this weapon for the Royal Norwegian Navy in the 1930s and was kept in limited production throughout the war. Introduced into German naval service about late 1943 and was used to arm the cruisers Admiral Hipper and Prinz Eugen as well as some Schnellbootes. As far as is known, only single mountings were ever used on warships and only HE tracer was issued. The Kriegsmarine received 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.
|Notes on Japanese Weapons
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.
31 August 2008 - Benchmark
23 September 2009 - Corrected typographical error, added comment about front-line service life
01 November 2010 - Corrected range table for USN M81A1
11 January 2011 - Added data reference
06 April 2011 - Added data reference
07 June 2011 - Additional information about usage by Kriegsmarine
14 July 2011 - Added crew information for USN Twin and Quad Mounts
28 December 2013 - Minor additions
06 February 2016 - Additional details on how the gun fired, Lend-Lease distribution, added Bofors ammunition details and gun production, added information on British ammunition, mountings and weapons