Bofors 40 mm/60 (1.57") Model 1936
United States of America
40 mm/56 (1.57") Mark 1, Mark 2 and M1
40 mm/56.3 (1.57") QF Marks I, III, IV, VIII, IX, X, XI, NI and NI/I
4 cm/56 (1.57") Flak 28
4 cm/60 (1.57") Type 5

Updated 16 November 2006

Probably the best heavy MG AA weapon of World War II, Bofors guns of this type are still in service even today.  This weapon was used on almost every major US and UK warship of World War II and was a very potent AA gun.  The Germans used Norwegian-produced Bofors guns which they designated as the 4 cm/56 Flak 28 and the Japanese copied a British Army air-cooled Bofors captured at Singapore to produce their 4 cm/60 Type 5.

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 the Krupp version.  What became the 40 mm Bofors was first prototyped in 1933 but it was the Model 1936 which was adopted for production.

The British Army first showed interest in these guns in 1933 and placed an order for 100 of these weapons 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 from 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 USN 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 US observers in a test off Trinidad.  BuOrd formally obtained Swedish licenses in June 1941, although some manufacturing actually started prior to that time.

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 from 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.  Third, all 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 guns were air-cooled, limiting their 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 metric.  To give one additional example of the design differences made for US 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 (340,200 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 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.

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, with a total of about 39,200 being built by the end of the war.  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 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, so 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.

All early versions of this weapon used friction-coupled drives, which quickly wore out on naval ships due to salt contamination.  Later versions built in the USA 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 M1.

These guns are recoil operated and use a monobloc barrel with a detachable breech ring, breech casing and automatic loader.  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 in 1945
U.S. Naval Historical Center Photograph # 80-G-413915

Click here for additional pictures
Gun Characteristics
Designation Sweden:  Bofors 40 mm/60 (1.57") Model 1936
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
German:  N/A
Japan:  1943
UK:  1941
USA:  1941
Date In Service Sweden:  N/A
Germany:  1944
Japan:  Not in service
UK:  1941
USA:  1942
Gun Weight German:  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:  about 1,150 lbs. (522 kg) [water-cooled]
Gun Length oa German:  N/A
Japan:  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)
Bore Length
(see Note 8)
German:  N/A
Japan:  94.5 in (2.400 m)
UK:  88.578 in (2.250 m)
USA:  88.6 in (2.250 m)
Rifling Length German:  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)
Grooves German:  N/A
Japan:  (16)  0.0098 in deep (0.25 mm)
UK:  (16) 0.236 in deep x 0.220 (0.60 x 5.59 mm)
USA:  (16) 0.0236 in deep x 0.220 (0.60 x 5.59 mm)
Lands German:  N/A
Japan:  N/A
UK:  0.0892 in (2.66 mm)
USA:  0.0892 in (2.66 mm)
Twist German:  N/A
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 dm3)
USA:  28.3 in3 (0.464 dm3)
Rate Of Fire
(see Notes)
120 rounds per minute per barrel nominal
140 to 160 rounds per minute when horizontal (gravity assist)

1) The practical rate of fire for clip-fed water-cooled guns is essentially the same as the cyclic rate, as the clips allow for a continuous ammunition feed and the recirculating water jackets keep the barrels from over-heating.

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) Mark 1 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.

5) The gun loader feed guides normally held eight rounds (two clips), although ten rounds could be loaded with two loose rounds between clips.  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 allows firing to be quickly resumed when a new clip is 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 eight rounds were fired, then the first loader would have to start from the beginning to drop in two clips and then 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 at 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) 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.

Type Fixed
Projectile Types and Weights Bofors
   AB 40 mm L/60 HE-T - 2.05 lbs. (0.93 kg)

   HE Mark 1 - 1.985 lbs. (0.900 kg)
   HE Mark 2 - 1.985 lbs. (0.900 kg)
   AP M81A2 - 1.960 lbs. (0.889 kg)

   HE - 1.970 lbs. (0.894 kg)
   SAP - N/A

   HE - 2.105 lbs. (0.955 kg)

   HE - 2.21 lbs. (1.002 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)
Others - N/A
Bursting Charge Bofors (modern day)
   0.20 lbs. (0.092 kg) Hexotonal

USA (World War II)
   HE Mark 1 - 0.148 lbs. (0.067 kg) TNT
   HE Mark 2 - 0.150 lbs. (0.068 kg) TNT
   AP - None (Solid Bullet)


Projectile Length USA HE Mark 1 and Mark 2 - 7.25 in (18.4 cm)
Others - N/A
Complete Round Length Bofors - 17.60 in (44.7 cm)
UK - 17.75 in (45.1 cm)
USA - 17.62 in (44.75 cm)
Others - N/A
Propellant Charge USA - 0.694 lbs. (0.314 kg) NC 025
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)
Cartridge 40 x 311 mm
Muzzle Velocity
(see Note 4)
USA Marks 1 and 2:  2,890 fps (881 mps)
UK Mark NI:  2,720 fps (829 mps)
Germany Flak 28:  2,801 fps (854 mps)
Japanese Type 5:  2,953 fps (900 mps)
Working Pressure
(see Note 4)
Japan - 16.50 tons/in2 (2,600 kg/cm2)
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 German
   Admiral Hipper and Prinz Eugen:  2,000 rounds
   German S-Boats:  500 rounds

   HMS Vanguard:  1,269 rounds (including 34 practice)

   Most ships:  2,000 rounds

Others:  N/A


1) Ammunition for most guns is held in four-round 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) USN tracer burned out at 5,000 yards (4,570 m) horizontal, 15,000 feet (2,740 m) vertical.

3) 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.

4) In "US Naval Technical Mission to Japan report O-47(N)-2" it is stated that the muzzle velocity 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.

Range - USA
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 (9,358 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 (9.679 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)
   4,200 yards (3,840 m):  8.5 seconds
   4,500 yards (4,110 m):  10.5 seconds

Range - UK
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.
Range - Germany
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)
AA Ceiling 22,970 feet (7,000 m)
Range - Japan
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
(see Note)
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.
Armor Penetration with 1.960 lbs. (0.889 kg) AP Shell
Estimated for "Class B" Homogeneous Armor
"Class B" Armor
0 yards (0 m)
2.70" (69 mm)
2,000 yards (1,829 m)
1.20" (30 mm)
4,000 yards (3,658 m)
0.60" (15 mm)
6,000 yards (5,486 m)
0.45" (11 mm)
Note:  This data is from "Battleships:  United States Battleships 1935-1992" and is based upon the USN Empirical Armor Penetration formula.
Mount / Turret Data
USA-Built Mountings
Mark 1 Twin
Mark 2 Quad
Mark 3 Single
Mark 4 Quad
(no shield)
9,800-13,000 lbs. 
(4445-5897 kg)
23,200-23,800 lbs.
(10,524-10,796 kg)
2,440-4,200 lbs.
(1,107-1,905 kg)
22,795 -24,553 lbs. 
(10,340-11,137 kg)
-15 / +90 degrees
-15 / +90 degrees
 -6 / +90 degrees
-15 / +90 degrees
Elevation Rate
24 degrees / second
24 degrees / second
 See Note 1
55 degrees / second
360 degrees
360 degrees
 360 degrees
360 degrees
Train Rate
26 degrees / second
26 degrees / second
See Note 1 
50 degrees / second
Recoil 8 - 9 in (20 - 22 cm)
Notes on USA-built Weapons and Mountings

1) Mark 3 single mounts used air-cooled guns, which were modified versions of the US Army M1 Bofors gun.  Four 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).  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 used rebuilt M1 guns and 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.

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 at 100 guns in 25 quad mounts.  Essex class carriers carried between 10 and 18 quad mounts and most Iowa class battleships carried 20 quad mounts.

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.  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.  Instead, they were given letter suffixes, the specifics of which I lack at this time, although it appears that the Mark IV was Type D for the left gun and Type E for the right gun.  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.

2) The USA provided 393 each of their Mark 1 (left-hand) and Mark 2 (right-hand) guns.  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, 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 control 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 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.'

5) 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.  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.  The Mark V proved quite popular in service and was retained 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.

6) The Mark VI was a sextuple mount using the Mark IX gun and used a 36-round ammunition tray for each gun rather than the usual four-round clips.  Training and elevating speeds of the RP50 Mark VI were both 30 degrees per second.  Weight was 21.24 tons (21.58 mt).  This mount did not enter service until after the war.

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 design 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 and 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 derived from 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.

12) The Buster 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 cancelled.  Used the Mark VIII gun.

Notes on Australian Weapons and Mountings

Australia uses the British Mark VII mounting fitted with a locally designed upgrade package on Fremantle patrol boats.  Elevation is -5 / +90 degrees and train is 360 degrees.  Training speed of 20 degrees per second and elevation speed of 40 degrees per second.

Notes on South Korean Weapons and Mountings

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 and Mountings

These 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.

Notes on Japanese Weapons and Mountings

The Type 5 (Model 1945) originated from the capture of a British Bofors air-cooled gun in a single hand-worked mounting at Singapore in 1942.  A Japanese copy underwent prototype firing trials in 1943 at the Torigasaki range of the Yokosuka Naval Arsenal and 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 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.

Notes on Soviet Weapons and Mountings

Some 700 Naval Model Bofors guns and 680 Army Model Bofors guns manufactured in the USA were sent to the Soviet Union as a part of Lend-Lease.

Data from
"Naval Weapons of World War Two" by John Campbell
"Joining the War at Sea" by Franklyn E. Dailey Jr., Capt. USNR (Ret.)
"The Illustrated Encyclopedia of 20th Century Weapons and Warfare" by Bernard Fitzsimmons
"US Battleships:  An Illustrated Design History," "US Naval Weapons" and "The Naval Institute Guide to World Naval Weapon Systems 1991/92" all by Norman Friedman
"German Warships 1815-1945 Volume I and Volume II" by Erich Gröner
"Small Arms, Artillery & Special Weapons of the Third Reich" by Terry J. Gander
"Jane's Ammunition Handbook:  Ninth Edition 2000-2001" edited by Terry J. Gander and Charles Q. Cutshaw
"Destroyer Weapons of World War 2" by Peter Hodges and Norman Friedman
"Radar at Sea" by Derek Howse
"U.S. Navy Bureau of Ordnance in World War II" by Lt. Cmdr. Buford Rowland, USNR, and Lt. William B. Boyd, USNR
"Champions of the Pacific" articles in "Warship Volume II" by Lawrence Sowinski
"Iowa Class Battleships" by Robert F. Sumrall
"German Cruisers of World War Two" and "German Coastal Forces of World War Two" both by M.J. Whitley
"Naval Ordnance and Gunnery - 1952" Navpers 16116-B
US Naval Technical Mission to Japan report O-47(N)-2:  Japanese Naval Guns and Mounts - Article 2, AA Machine Guns and Mounts
Special help from Mark Fitzpatrick, Robert Hurst, Cliff McMullen, Robert Stoner and Vladimir Yakubov