1) The practical rate of fire for clip-fed water-cooled guns comes close to 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 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.

USA
   HE Mark 1 Mods 1 to 24 - 1.985 lbs. (0.900 kg)
   HE Mark 2 Mods 1 to 35 - 1.985 lbs. (0.900 kg)
   AP M81A1 and M81A2 - 1.960 lbs. (0.889 kg)

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

Germany
   HE - 2.105 lbs. (0.955 kg)

Japan
   HE - 2.21 lbs. (1.002 kg)

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

Others
   N/A

UK
   HMS Vanguard:  1,269 rounds (including 34 practice)
   Standard outfit for most ships was 1,564 rounds per gun

USA
   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) 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) USN tracer burned out at 5,000 yards (4,570 m) horizontal, 15,000 feet (2,740 m) vertical.

4) USN bourrelet diameter was 1.570 inches (39.88 mm).

5) 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.

6) 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.

7) 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.

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.

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

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.  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 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.

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.

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 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, 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 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.'

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 (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 cancelled.  Used the Mark VIII gun.

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.  Training speed of 20 degrees per second and elevation speed of 40 degrees per second.

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.

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.

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.

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.

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.