Bofors 40 mm L/60 gun

Contrary to the name, the Bofors 40 mm Automatic Gun L/60 has an actual barrel length of 56.25 calibres – 2,250 mm (7 ft 5 in). Due to this, the gun can at times be found under the designation L/56.[18] The L/60 calibre length – 2,400 mm (94 in) – refers to the length of the barrel and breech mounted as a unit.[19] The gun has also been cited with a caliber length of L/62 on occasion. This refers to the length of the barrel – 2,250 mm – when equipped with the conical flash hider – 250 mm (9.8 in); amounting to a length of 2,500 mm (8 ft 2 in), or 62.5 calibres.[20]

Similarly, the shorter L/43 calibre length – 1,720 mm (5 ft 8 in) – refers to the length of the barrel and breech mounted as a unit.[20] The actual barrel length is 1,570 mm (5 ft 2 in) – 39.25 calibres.[20] With a flash hider or muzzle brake mounted the barrel length becomes 1,740 mm (5 ft 9 in)[20] or 1,760 mm (5 ft 9 in).[21]

Within a few years AB Bofors received number of orders, but more importantly at that time, a number of foreign governments negotiated for licensed production of the gun and its ammunition. These nations included Poland, Finland, Greece, Norway and many other countries. Thus by 1939 Bofors gun was in production all over Europe for many armies in bewildering arrangement of cross-deals. For instance, the United Kingdom took out a license, but was in such a hurry to re-arm with the Bofors gun that it also purchased quantities from Poland and Hungary. France wanted to set up a line but purchased guns from Poland. Poland meanwhile developed its own version of Bofors gun, contributing a lighter carriage (in the 40-mm armata przeciwlotnicza Bofors wz. 36) which was later adopted by the British.

The Swedish navy adopted the weapon as the m/36 in two versions: hand-worked single air-cooled, and power-operated twin water-cooled. A twin air-cooled mounting, probably hand-worked, was also used by the navies of Sweden and Argentina, and a twin air-cooled wet mounting was developed for Polish submarines.

Q.F. 40 mm Mk. 1 displayed at CFB Borden. This example mounts a Stiffkey Sight, and displays the additional armor protecting the gunners

Bofors gun on the Ley-class minehunter HMS Isis (1978)

The British Army had first examined the weapon in 1937, when it received a number of Polish-built examples for testing. They were known as the "QF 40 mm Mark I" (QF standing for "quick firing"). With a minor change to the flash hider, they were designated "Mark I/2". A production licence was acquired, and the gun was converted from metric to imperial measurements. Numerous changes were made to the design so that it was more suitable for mass production, because the original Bofors design was intended to be hand-assembled. Many parts were labelled "file to fit on assembly", requiring many man-hours of work to complete.

Testing showed that there was a serious problem with aiming the gun at high-speed aircraft. Although it could be trained quickly, aiming accurately while doing so proved difficult. In order to deal with that, the British introduced a complex mechanical analogue computer, the Kerrison Director, which drove the laying electrically. A three-man team operated the director by pointing it at the target whilst dialing in estimates for speed, range, and various atmospheric conditions. The director then aimed the gun using powered mounts, while a gunner loaded the clips. That eliminated the need for the lead-correcting reflector sights, which were replaced with a backup system consisting of a simple ring-and-post sight, known as a "pancake".

In that form, the "QF 40 mm Mark III" (Mk II was a designation used for a version of the naval "pom-pom" anti-aircraft gun), became the army's standard light anti-aircraft (AA) weapon, operating alongside their 3-inch 20 cwt and 3.7-inch heavy AA guns. British production started slowly and, by September 1939 only 233 equipments had been produced. However, by the end of the war, the total production of British, Canadian and Australian factories numbered over 19,000. The peak production year was 1942, when British factories produced 5,025 and Canadian factories produced 1,311.[24]

Bofors L/60 sighting system, left side

In combat, it was found that the Kerrison was difficult to set up in many situations, as well as making logistics more complex, due to the need to keep its electrical generator supplied with fuel. In most engagements, only the pancake sights were used, without any form of correction, making the British versions less capable than those used by other forces. Eventually, an anti-aircraft gunnery school on the range at Stiffkey on the Norfolk coast, delivered a workable solution, a trapeze-like arrangement that moved the pancake sights to offer lead correction, operated by a new crew-member standing behind the left-hand layer. The "Stiffkey Sight" was sent out to units in 1943, arriving in Canadian units in the midst of the Battle of the Aleutian Islands. A final wartime change to the elevation mechanism resulted in the "QF 40 mm Mark XII". A much lighter, two-wheeled carriage was also developed for airborne use.

The army experimented with various self-propelled anti-aircraft guns based on various tank chassis. Changes to the breech for that role created the "QF 40 mm Mark VI", which was used on the Crusader tank to produce the Crusader III AA Mark I. The main self-propelled version of the Bofors was the gun mounted on a chassis derived from the Morris C8 "Quad" artillery tractor, which was known as the "Carrier, 30 cwt, SP, 4×4, 40 mm AA (Bofors)" or Morris-Commercial C9/B. Such guns were used in support of army divisions, to provide swift protection against air attack without the need to unlimber. They saw service in north-west Europe, where six SP Bofors of 92nd (Loyals) Light Anti-Aircraft Regiment, Royal Artillery, landed with the British 3rd Infantry Division on Sword Beach on D-Day, to protect the vital bridges over the Caen Canal and the Orne River (Pegasus Bridge and Horsa Bridge), shooting down 17 German planes. Later in the campaign, SP Bofors were used extensively for ground shoots as well as in an anti-aircraft role.

In British Army service, the Bofors found a highly specialised role: during the North Africa campaign, at the Second Battle of El Alamein, they were used to fire tracer horizontally to mark safe paths for units through the German minefields. That practice was further developed during operations in north-west Europe, where bursts of colour-coded tracer were used to define the axis of advance of different formations in large-scale night attacks.

The RAF Regiment was formed in February 1942, in response to the German capture of airfields with airborne troops in the Battle of Crete, which resulted in strategic defeat on the island by numerically inferior German forces. The formation of a dedicated airfield defence force included low-level air defence, in which the Bofors L60 — the same design as the Army version — was the principal weapon for the RAF Regiment's Light Anti-Aircraft squadrons in North Africa, Malta, Italy, the Balkans, the UK (including the allocation of fifty-two squadrons to Operation Diver defence against V-1 flying bombs in southern England), and north-western Europe (from Normandy landings through to the cessation of hostilities).

No 2875 Squadron RAF Regiment, employing the L60, became the first unit to shoot down a jet aircraft, a Messerschmitt Me 262, with ground-based anti-aircraft fire, at Helmond in the Netherlands on 28 November 1944. Although the Allied air forces had achieved air superiority by the Normandy landings, Advanced Landing Grounds continued to be high-priority targets for the Luftwaffe when the opportunity presented, and that ensured that the RAF Regiment's L60s continued to be heavily used. For example, on New Year's Day 1945, during the Battle of the Bulge, RAF Regiment Light Anti-Aircraft squadrons shot down 43 German aircraft and damaged 28 others during the Operation Bodenplatte attacks on eleven RAF forward airfields. There were insufficient guns available to equip the RAF Regiment squadrons in the Far East during the war, and they had to make do mostly with 20 mm Hispano and Oerlikon 20 mm guns.

A Light Anti-Aircraft field regiment (one with each infantry division) had 54 Bofors guns.[25]

After World War II, the RAF Regiment continued to employ the L60 as its principal anti-aircraft weapon until it was replaced by the L70 gun in 1957. The guns were deployed in the UK, Germany, Cyprus, the Middle East, and the Far East.

Single Bofors 40 mm Boffin mounting, displayed at CFB Borden. In 1996, museum pieces like this cannon were pressed back into service as the main armament of the Kingston-class minesweepers[26]

The Royal Navy also made extensive use of the Bofors. Its first examples were air-cooled versions quickly adapted for ships during the withdrawal from Norway. After the German invasion of the Netherlands in May 1940, the Dutch minelayer, HNLMS Willem van der Zaan, gave the navy its first example of a water-cooled gun on its Hazemeyer tri-axially stabilized mounting. Locally produced examples started arriving in 1942, known as the "QF 40 mm Mark IV" with twin-mounts, or the "QF 40 mm Mark V" with single mounts. The navy ran through a variety of versions of the basic Bofors gun over the war, including the Mark VII to Mark XI. The British light anti-aircraft weapon already in use with the Royal Navy's, the QF 2-pounder gun, also had a calibre of 40 mm, but was referred to as the QF 2-pdr.

In the Royal Navy guns and mounts were designated separately. The following mountings were used:

• Mark I: twin mounting based on American design and using American-built guns, not widely fitted. Fitted for remote fire control.
• Mark II: quadruple version similar to the Mark I
• Mark III: a navalized version of the Army single mounting, hand worked elevation and training.
• Mark IV: a tri-axially stabilized twin mounting, copied from, and usually known as, the "Hazemeyer". It had on-mounting fire control, and was usually fitted with Radar Type 282 to provide target range information.
• Mark V: twin mounting, which superseded, and eventually replaced, the Mark IV, often referred to as the "utility" mounting. It was a simplified, unstabilised mounting based on the American twin mounting Mark I, and was designed for remote fire control.
• Mark VI: a six-barreled weapon, feeding from large trays instead of clips, and designed for remote control from a dedicated radar-equipped director.
• Mark VII: a single-barreled, hydraulically-powered mounting, that superseded the Mark III and entered service in 1945.
• Mark IX: Mark VII mount modified for electrical power, as the Mounting Mark IX and, in that form, saw service in the Falklands War.

The Mounting Mark V (Mark VC for Canadian built examples) for the 20 mm Oerlikon and QF 2 pounder guns was also adopted initially as an interim mount for the Bofors. It was a single-barrelled mounting with hydraulic power, and was known as the "Boffin".

The final British Bofors mounting to see service was the "Stabilized Tachymetric Anti-Aircraft Gun" (STAAG), which was twin-barrelled, stabilised, and carried its own tachymetric (i.e. predictive) fire control system, based around the centimetre Radar Type 262, capable of "locking on" to a target. The mounting was heavy (17.5 tons) and the high-vibration of the gun mounting made it a poor location for sensitive valve electronics and mechanical computers. The STAAG Mark I carried the radar dish over the gun barrels, where it was subject to damage during firing so, on the STAAG Mark II, the set was shifted to the roof of the control cabin. STAAG was ultimately too difficult to maintain in the harsh environment of a warship and was replaced by the Mounting Mark V, with the fire control equipment located remotely, then by the single Mark VII and, ultimately, with the GWS20 Seacat missile system. The final version of STAAG was fitted to the RN Type 12 Whitby-class anti-submarine frigates and the Type 41 Leopard anti-aircraft and Type 61 Salisbury air direction frigates, completed from 1956 to 1958. The cruiser Royalist was also fitted with the STAAG Mk 2 during modernisation before transfer to the RNZN in the 1950s. Initially, it had three STAAG CIWS, the STAAG in Q position was removed in 1960, but it carried two mounts until the end of its service in Southeast Asian waters, in 1965. In 1956 during the Suez Crisis, the crew of Royalist seemed to find the STAAG a reasonably effective anti-aircraft weapon in Operation Musketeer.

In order to supply both the U.S. Army and U.S. Navy with much greater numbers of the guns, Chrysler built 60,000 of the guns and 120,000 barrels through the war,[1] at half the original projected cost, and filling the Army's needs by 1943.[27] Over the lifetime of the production, their engineers introduced numerous changes to improve mass production, eventually halving the overall time needed to build a gun. Most of the changes were in production methods rather than the design of the gun itself: for example, milling from steel block was replaced by stampings and castings whenever possible, and Amplex division of Chrysler (which normally manufacturated oilite) produced nine parts by sintering.[27][28] York Safe & Lock also produced the weapons, though its attempts to coordinate drawings across the program were unsuccessful, and this responsibility was transferred to the Naval Gun Factory in July 1943.[29]

There were many difficulties in producing the guns within the United States, beyond their complexity (2,000 subcontractors in 330 cities and 12 Chrysler factories were used to make and assemble the parts). The drawings were metric, in Swedish, with loose tolerances for hand fitting and read from the first angle of projection. Chrysler had to translate to imperial measures and English language, fix absolute dimensions, and mirror/reorder the drawings to the third angle of projection. Chrysler engineers also tried to simplify the gun, unsuccessfully, and to take high-speed movies to find possible improvements, but this was not possible until near the end of the war.[27]

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. Thirdly, 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. Fourthly, 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.[4][29]

A MK 12 quadruple mount of Bofors guns fires from USS Hornet

The United States Navy's Bureau of Ordnance purchased a twin-mount air-cooled example, spare parts and 3,000 rounds of ammunition directly from Bofors, which arrived in New York on 28 August 1940 aboard the Army transport USAT American Legion, which had evacuated 897 people, including members of the Norwegian royal family, through the Finnish port of Petsamo.[30] During that month another Dutch ship, the Van Kinsbergen, demonstrated the Hazemeyer mount to Navy observers. The gun was quickly chosen as the Navy's standard anti-aircraft weapon over the British 40 mm calibre, 2-pounder pom-pom; however, negotiations with Bofors for licensed production stalled when the Swedes requested airplane export and manufacturing licenses in return.[29] Reportedly, the Navy secretly imported a set of imperial designs from Britain and started production illegally. A formal contract with Bofors was reached in June 1941. The resulting Mark 1 and Mark 2 weapons were intended for the left and right side of a twin mount, respectively, and were adapted by Chrysler for water cooling.[27]

A twin Bofors 40 mm at Patriots Point

Following the attack on Pearl Harbor on 7 December 1941, the existing 1.1" (28 mm) quad mount and .50 caliber machine guns were determined to be inadequate against modern aircraft, and their replacement by 40 mm Bofors and 20 mm Oerlikon weapons was accelerated. The water-cooled version was used almost exclusively by the U.S. Navy and Coast Guard. The 40 mm quadruple mount was developed by essentially mounting two twin mounts side by side.[29] A major improvement was the addition of power operation to both twin and quadruple mounts. Essentially all US naval mountings were twin or quadruple. 40 mm weapons were eventually mounted on virtually every naval and armed auxiliary vessel larger than a small landing craft.[29] After the war, the 3"/50 caliber gun Mark 27 twin mount began to replace the Bofors, because the "VT" proximity fuse would not fit a 40mm projectile, and the 40 mm weapon was considered inadequate against the emerging anti-ship missile threat. The twin 3" mount was intended to be the same weight as the 40mm quad mount, but was somewhat heavier in practice, which had to be compensated for. Except on destroyers and new construction, the Navy was slow in phasing out the 40 mm gun, and it continued in active Navy service through the Korean War. It remained on inactive Reserve fleet ships at least through the early 1970s.[31]

The Navy's satisfaction with the weapons was demonstrated by their practice of telegraphing Chrysler Corporation with the serial numbers of guns when they shot down an aircraft.[27]

U.S. Army 40-mm Automatic Gun, Anti-Aircraft, M1, on 40-mm Gun Carriage M2(A1)[32]

In 1938 the United States Army introduced a 37 mm gun of their own design, but found it to be of limited performance. In early World War II, six British Bofors were imported for testing, along with Kerrison Predictor directors, and they proved to be superior in all areas. By the middle part of the war, most of the 37 mm guns had been replaced by the 40 mm.In U.S. Army and Marine Corps service, the single mount Bofors was known as the 40 mm Automatic Gun M1.[33] The U.S. version of the gun fired three variants of the British Mk. II high-explosive shell as well as the M81A1 armor-piercing round, which was capable of penetrating some 50 mm of homogeneous armor plate at a range of 500 yards. In the Army, each Anti-Aircraft Artillery (AAA) auto-weapons battalion was authorized a total of thirty-two 40 mm guns in its four firing batteries.[34][35] Each U.S. Marine division had a "special weapons battalion" that included sixteen 40 mm guns; in early 1944 these were replaced with anti-aircraft battalions with twelve 40 mm guns. Marine defense battalions also used the 40 mm gun.[36] All of these unit types also included other AA weapons.

During World War II, the twin mount version of the gun was mounted on an M24 Chaffee tank chassis as the M19 Gun Motor Carriage. In the 1950s, the M41 Walker Bulldog tank was heavily modified into the M42 Duster with the same twin 40 mm mounting. After being largely withdrawn from service in the early 1960s, the M42 was re-introduced beginning in 1966 for the Vietnam War, where it was mostly used for ground fire support. Following the withdrawal from Vietnam in the early 1970s, the M42 was retained in National Guard service until finally retired in 1988.

The Bofors 40 mm L/43 is a submarine variant of L/60 with retractable mounting, and using low propellant charge ammunition.[41][18][42]

The L/60 Bofors fitted to the Kingston-class coastal defence vessel HMCS Nanaimo

Twin L/60 dismounted from an Argentinian Drummond-class corvette which saw service during the Falklands War

Gunnery practice on the aircraft carrier HMAS Sydney off Korea in 1951

The L/60 remained in front-line service well into the 1980s and 1990s although it had replaced in production by the Bofors 40 mm Automatic Gun L/70, . In most cases, these were the ground anti-aircraft versions, as a suitable replacement in this role did not come along until the introduction of truly effective MANPADS missiles in the 1980s and 1990s.

In United States Army service, the M19 Gun Motor Carriage was replaced by the M42 Duster, using the same turret but based on the chassis of the M41 Walker Bulldog tank.[43]

The L/60 saw active service with the Argentinian and British navies in the 1982 Falklands War and continued to be used into the 1990s, when it was replaced by modern 20 mm and 30 mm artillery.

The Canadian Forces removed the guns from their surface fleet in the late 1980s when they were considered to be outdated, only to re-use old Bofors guns as the main armament of the Kingston-class coastal defence vessel.[26] The Bofors served as the main armament for almost 20 years.[44] The decision to remove them was made in 2014, due to their maintenance burden, and their lack of stabilization.

As of August 2006, the French navy uses L/60s on more than twenty ships (patrols and auxiliaries).

Ships of the Norwegian and Icelandic Coast Guards continue to use the 40mm Bofors gun.

The L/60 continued in use in the Irish Army until recent years, when it was retired in favour of the radar-controlled L/70. The Irish Naval Service P20 class retained L/60s on board as their main weapon until the 1990s but were rearmed with L/70s. The last remaining P20-class patrol vessel, (LÉ Aisling) decommissioned in 2016, was the final vessel fitted with the L/70. Two retired L/60s can be seen adjacent the square in Sarsfield Barracks, Limerick.

The last 40 mm L/60 Bofors in service with the Royal Australian Navy (RAN) were used as the main weapon aboard the Attack class and Fremantle class patrol boats and for training purposes at the West Head gunnery range at HMAS Cerberus.[45] These were removed from service during 2007; Bofors were used aboard almost every RAN ship to operate between the 1940s and the 1990s, including the aircraft carriers Sydney and Melbourne.[45]

In 2012, the L/60 was still being used by Brazil, Indonesia, Paraguay, Taiwan, and the United States.

Bofors guns on an AC-130A Spectre gunship

Since the beginning of the 1970s Bofors L/60s have been used in the United States Air Force's Lockheed AC-130 gunships in the air-to-ground role.[46] Between 2006 and 2012, there were plans to remove these and the M61 Vulcans from newer AC-130U variants and replace them with 30 mm autocannons. However, these plans did not come to fruition at the time, and the Bofors and Vulcans remained in service as of 2009,[47] though the later W- and upcoming J-model variants have 30 mm Mk44 Bushmaster II autocannons instead.[48][49]

When four additional AC-130Us were to be converted from 2002, the necessary 40 mm L/60 guns had to be salvaged from old M42 targets at the Nellis AFB range.[50] The final 40 mm L/60 guns in US service were retired in 2020 with the last of the AC-130Us.