The Bofors Gun – Part II: the L/70 Weapons

By the end of World War II in 1945, the advent of turbojet-powered warplanes had increased the speed of attack so greatly that the Bofors L/60 gun could not possibly loft sufficient rounds to counter warplanes between the time they were spotted, made their attacks and flew out of range. In order to counter these threats, the gun needed both longer range and a higher rate of fire, thereby increasing the number of rounds fired over the period of an engagement. Bofors considered either updating the 40-mm weapon or creating a much more powerful 57-mm design, but finally did both.

The new 40-mm gun with an L/70 rather than L/60 barrel was designed to use a 40 × 364R round, larger than the L/60 weapon’s 40 x 311R round, and the larger round fires a slightly lighter 30.69-oz (870-g) projectile at the much higher muzzle velocity of 3,379 ft (1030 m) per second. The rate of fire was increased to 240 rounds per minute, and the carriage was modified to allow powered laying, the power for the latter being supplied by a generator placed on the front of the carriage. Further improvements over the years have improved the firing rate first to 300 and then to 350 rounds per minute.

The weapon entered Swedish service in 1951, and foreign sales started, as they had with the L/60 weapon, with the Netherlands and UK. In November 1953 the L/70 weapon was accepted as the NATO standard anti-aircraft gun, and was soon produced in the thousands. The L/70 was also used as the basis for a number of self-propelled anti-aircraft gun mountings, notably the US Army’s M42 Duster and proposed M247 Sergeant York.

Bofors 40 mm L70 AA gun at an Israeli airbaseDeveloped after the end of World War II, and thus incorporating many of the lessons learned the hardest possible way about point defence against low-level air attack, the Bofors Ivakan m/48 40-mm single AA gun mounting was first produced in prototype form in 1947 and began to enter service with Sweden in 1951. The weapon is now known generally as the Bofors 40L70 and marks the high point in a process of continuous development since 1928. The Bofors 40L70 remains one of the most powerful light weapons in the world, operating in both the surface-to-air and surface-to-surface roles. This performance results from the weapon’s high rate of fire, good range and excellent ammunition. It is found on four different carriages; namely the m/48, m/48C, m/48D and m/48R, of which the last two can be used with a radar-directed fire-control system.

The type has been built under licence in India, Italy, the Netherlands, Spain, the UK and what was then West Germany, and was also developed on special mountings notably by Breda in Italy (and licence-built in Spain). The USA has also adapted the type for air-to-surface use as part of the prodigious armament fit of its Lockheed Martin AC-130 ‘Spectre’ aerial gunships.

Basic mounting
The equipment was developed primarily for clear-weather daylight use, but was also extensively developed for adverse-weather and night capabilities fully matching the requirements of the modern battlefield. The basic mounting comprises a two-axle carriage that can be towed by any suitable 4 x 4 or 6 x 6 3-ton truck, on which is mounted a turntable for the gun mounting. The turntable provides 360° traverse, and in firing position the whole assembly is supported by four stabilizing jacks, located one at each end of the carriage, and one at each of the lateral outriggers, with its wheels lifted off the ground. The on-mounting crew is four, comprising laying and elevating numbers seated to the left and right of the breech mechanism, and two loading numbers standing at the rear of the mounting. Despite its comparatively large calibre for an automatic cannon, the Bofors 40L70 has a usefully high rate of fire, resulting largely from the use of an automatic breech combined with the ejection of empty cases towards the end of the recoil, the cases being deflected into a chute at the front of the mounting, and the ramming of the fresh round during the run out.

Ammunition is fed to the breech by an automatic overhead loader that is filled with ammunition from four-round clips supplied from the equipment’s ready-use ammunition capacity of 96 rounds. These are stored in two racks at the rear of the gun mounting beside the two loaders, who each stand inside a waist-high ring to prevent them being thrown off the mounting at high rates of traverse. The racks are replenished as required by an external loading party. The capacity of the automatic loader can be increased to 26 by the use of a tall ammunition stay, providing useful sustained fire capability when the mounting is used in its remote-control mode, with laying/elevating data and fire instructions provided via cables from a fire-control system.

Ammunition options
The Bofors 40L70 can fire a wide assortment of ammunition produced in several countries, but the definitive range is still that made in Sweden by Bofors. The Bofors range of 40-mm ammunition encompasses five primary types, all 21.02 in (534 mm) long: APC-T, HCHE, HE-T, PFHE and TP. The APC-T (Armour-Piercing Capped – Tracer) round weighs 5.51 lb (2.5 kg) and fires its 32.45-oz (920-g) shot with a muzzle velocity of 1010 m (3,314 ft) per second. The HCHE (High-Capacity HE) round is a good example of the new type of multi-role ammunition and weighs 5.29 lb (2.4 kg), firing its 30.69-oz (870-g) projectile with a muzzle velocity of 3,379 ft (1030 m) per second. The projectile contains 5.82 oz (165 g) of octonal explosive inside a casing of specially hard steel sufficient to allow penetration of 0.79-in (20-mm) armour without disintegration. The casing also protects the delay-action fuse of the projectile, ensuring detonation only after the target has been penetrated to maximise damage inflicted on targets such as armoured personnel carriers. The HE-T (High Explosive – Tracer) round weighs 5.51 lb (2.5 kg) and fires its 33.86-oz (960-g) projectile with a muzzle velocity of 3,297 ft (1005 m) per second. It contains 3.63 oz (103 g) of hexotonal explosive. The TP round has the same performance and ballistic qualities as the HE-T round.

The most important round, however, is undoubtedly the PFHE (Pre-Fragmented HE) round. This weighs 5.29 lb (2.4 kg) and fires its 31.04-oz (880-g) projectile with a muzzle velocity of 3,363 ft (1025 m) per second, producing flight times of 1.1 seconds to 1,095 yards (1000 m), 2.44 seconds to 2,185 yards (2000 m) and 4.44 seconds to 3,280 yards (3000 m). The projectile contains 4.23 oz (120 g) of octonal explosive inside a pre-fragmented casing that shatters into 2,400 or more fragments. This spreads over a considerable volume, as a result of the shape of the projectile’s boat tail, to increase the probability of a hit on a key component and thus deliver a decisive wound to the target. Moreover, the casing of the projectile includes 650 tungsten carbide pellets which possess a lower-drag shape than most fragments and, as they lose their velocity less rapidly, have better short-range armour-penetration capability, in this instance 0.55 in (14 mm) of Duralumin. The PFHE round is fitted with a pulse-Doppler radar proximity fuse, ensuring detonation between 16 ft 5 in and 23 ft 0 in (5 and 7 m) from an aeroplane, and 14 ft 9 in (4.5 m) from a missile. This latter distance was found to be too great for effective use of the PFHE round against sea-skimming missiles, and a later PFHE Mk 2 projectile thus has an improved fuse with a trigger distance of 3 ft 3 in (1 m) to make possible its use against targets operating at a height below 16 ft 5 in (5 m). This reduced trigger distance also increases the density of the fragmentation pattern hitting the target, and so improves kill probability.

Fast traverse and elevation
The Bofors 40L70 is electro-hydraulically controlled, traverse and elevation rates of 85° and 45° per second being achieved. There is also a manual laying system for use in the event of a primary power system failure. When used in local control, the Bofors 40L70 is commanded by a gunner located to the left of the breech mechanism, using a small control column to move the weapon in traverse and elevation. If this system is disabled, the manual controls for laying and elevating are used by the operating numbers to the left and right of the breech.

The basic sight system for the Bofors 40L70 comprises two identical NIFE SRS-5 optical sights located over the breech for use by the laying and elevating numbers. Though fitted with its own on-mounting sights, therefore, the Bofors 40L70 is best used when tied into a higher-level target-detection and fire-control system to produce a more capable overall air-defence outfit. Apart from the Ericsson PS-04/R used by the Swedish army, the off-carriage fire-control systems most commonly associated with the Bofors 40L70 are two Dutch systems (the Hollandse Signaalapparaten Flycatcher and older L4/5); one Franco-Italian system (the Thomson Houston/Officine Galileo Eldorado); and two Swiss systems (the Contraves Skyguard and older Contraves Super-Fledermaus).

Radar-aided sighting
Formally designated the HSA VE4/41, Flycatcher is designed for the detection, identification, tracking and engagement of medium-, low- and very low-level air attacks with guns and/or SAMs. It is thus a highly versatile system, and uses a radar derived from that used in the Dutch army’s 5PFZ CA1 Caesar version of the multi-national 5PFZ self-propelled AA cannon mounting. The Flycatcher system weighs about 4,409 lb (2000 kg) and is accommodated in a container 8 ft 11.5 in (2.73 m) long, 6 ft 11.5 in (2.12 m) wide and 6 ft 11.75 in (2.13 m) high increasing to 11 ft 11.75 in (3.65 m) high with the dual search-and-track antenna system raised. The pulse-Doppler radar system operates I-band for the detection of targets out to a 360° range of some 21,875 yards (20000 m). Possible targets are interrogated by the system’s integral IFF sub-system, and is tracked in the I/K-bands with data displayed on the operator’s north-oriented plan position indicator display. The operator designates a target, which is then automatically acquired and tracked as the digital fire-control system solves the fire-control problem and lays the relevant weapon onto the right bearing and elevation. The system is capable of engaging targets moving at up to 972 kt (1,118.5 mph; 1800 km/h) with vertical speeds of up to 985 ft (300 m) per second; the associated weapon opening fire as soon as the right engagement criteria have been established.

The system also possesses a TV camera boresighted to the radar, allowing a useful passive back-up tracking capability should the radar have to be shut down for fear of anti-radiation missiles. The radar has extensive ECCM capabilities, and other anti-interference features. The L4/5 was developed in the early 1960s specifically for the control of 40-mm batteries of up to three guns, but can also be adapted for the control of surface-to-air missiles.

Operated by a crew of two or three, the complete L4/5 weighs about 13,228 lb (6000 kg) and is carried on a two-axle trailer that is jack-stabilised when the system is operating. Dimensions for the complete unit include a length of 24 ft 7.25 in (7.50 m), width of 7 ft 2.625 in (2.20 m) and height of 10 ft 10 in (3.30 m) increasing to 17 ft 0.75 in (5.20 m) with the radar unit erected through 90° to its vertical operating position.

The central electronics pedestal is surmounted by a truncated conical radome covering the two search and single tracking antennae, the former being back-to-back equipments designed for the medium/low- and very low-altitude search altitudes. The system possesses a TV back-up tracking facility, and has a track-while-scan capability to a range of 32,810 yards (30000 m). The system possesses a moving target indication capability, and the two operators, located in an extending shelter at the rear of the system, can use 360° or sector search followed by an automatic or manual target tracking. The reaction time of the system is fast, principally because a high-speed digital computer is used as the core of the data-processing function.

Franco-Italian development
The one-man TH.D.1229 Eldorado system was developed jointly by Thomson Houston (later Thomson-CSF) and Officine Galileo, the former being responsible for the radar and its servo mechanisms, and the latter for the trailer, computer and displays. The trailer is a single-axle unit weighing some 3,527 lb (1600 kg) complete with displays, computer and roof-mounted unit accommodating the optical tracker and antenna for the acquisition radar. It is designed for use in conjunction with the Thomson Houston TH.D.1350 Mirador II specialised target-acquisition radar, whose 114.6-lb (52-kg) combined antenna/tripod unit can be carried on the trailer, though designed for free-standing erection. The Mirador II operates in S-band, and is a coherent pulse-Doppler track-while-scan equipment used for the 360° acquisition and tracking of low-level targets out to a range of 19,685 yards (18000 m). The system is fitted with an audio alarm, and moving target indication data are shown on the operator’s plan position indicator display in the trailer.

The Contraves Skyguard fire-control system was designed for the control of powerful AA batteries equipped with guns or missiles, or alternatively blending the capabilities of a mixed defence using heavy cannon and short-range missiles. The system is based on the same two-axle chassis as the Oerlikon-Bührle GDF twin 35-mm towed AA gun mounting often associated with the Skyguard system, and is essentially a container-mounted system with an overhead arrangement of sensor heads. It entered production in 1975 as successor to the highly popular Super-Fledermaus equipment, providing more modern hardware, separate search and tracking radars, and digital rather than analog computing. The radar is provided by Ericsson of Sweden, and comprises a UAR 1021 I/J-band coherent pulse-Doppler search and tracking equipment with its antennae on a mounting about the mobile housing to provide search capability to a range of 21,875 yards (20000 m). A separate monopulse-Doppler tracking element provides tracking from a range of 12,250 yards (11200 m) down to 330 yards (300 m). The system offers high rejection of clutter and electronic countermeasures, together with moving target indication of multiple targets. The radar is backed by automatic closed-circuit TV tracking. Data are fed into a Contraves Cora II M computer, a real-time unit that undertakes the functions of system check, threat evaluation, and fire-control solution for guns and missiles.

Swiss system
The Super-Fledermaus is also a towed system, again based on a two-axle chassis with accommodation for the tactical crew inside an extending shelter at the rear of the trailer. The system weighs some 11,023 lb (5000 kg) and was designed for the control of three heavy-calibre guns or three SAM launchers. This system has a combined head for the visual tracker and Siemens-Albis search/tracking radar, allowing target detection by helical or sector scanning at ranges up to 54,680 yards (50000 m), followed by target tracking at ranges inwards from 43,745 yards (40000 m). The computer system allows for ambient conditions and gun parallax, and delivers a continuously updated fire-control solution, improved further during firing by virtue of the fact that three muzzle-velocity measurement sets are part of the equipment. The equipment can be operated in manual, semi-automatic and automatic modes. Additional features include the possibility of linkage into a higher-level detection system and the use of a putting-on capability to allow rapid redirection of the system should the spotting number detect an unexpected target.

Israel used the Super-Fledermaus system with its 40-mm Bofors guns, and in more recent years developed an improved version of this Swiss fire-control system as the MBT Weapon Systems Eagle Eye, otherwise known as the Simple Economic Fire-Control System. This comprises the Elta EL/M-2106 point defence alert radar combined with an upgraded version of the Super-Fledermaus fire-control system. The EL/M-2106 is a D-band equipment weighing only 178.6 lb (81 kg), its detection ranges against 21.5-sq ft (2-m² ) targets being 17,500 yards (16000 m) for fixed-wing aircraft and 10,935 yards (10000 m) for helicopters. The EL/M-2106 was also produced under licence in the USA as the Lear Siegler Sentry. The fire-control system has a new and considerably more powerful computer in the tracker, which is also improved with an optronic tracking mount fitted with a TV camera, an optical sight and a laser rangefinder. Provision was also made for subsequent ‘growth’ to a TV tracker and FLIR sensor. According to the manufacturer, Eagle Eye can control up to six 40-mm guns, and by comparison with the baseline Super-Fledermaus offers three times the engagement likelihood and two times the kill probability. Eagle Eye can also be used with other modern weapons of the larger calibre.

In Sweden the electronics company SATT developed a useful upgrade package that could be fitted to Super-Fledermaus as well as to other fire-control systems. The package is centred on a new fire-control computer with associated interfaces and an inbuilt trainer with 1,000 scenarios. The option of several interfaces allows the upgrade package’s incorporation with different radars for both search and tracking, a laser rangefinder and muzzle velocity measuring equipment. SATT claims that the upgrade package offers a five-second reaction time and an 88% hit probability with a 10-round burst at 2000 m (2,185 yards).

Specification

Bofors m/48 (40L70 Type B) 40-mm single AA gun mounting

Type: towed single point-defence tactical AA gun mounting

Calibre: 40 mm

Barrel length: 70 calibres

Carriage: four-wheel towed platform with outriggers and shield

Weight: 11,354 lb (5150 kg) in travelling order

Dimensions: length, travelling 23 ft 11 in (7.29 m); width, travelling 7 ft 3.5 in (2.23 m) and firing 14 ft 6.75 in (4.44 m); height, travelling 7 ft 8.5 in (2.35 m) and firing 6 ft 4.33 in (1.94 m)

Traverse/elevation: 360° at 92° per second/ -20° to +80° at 57° per second

Rate of fire: 300 or (in later models) 330 rounds per minute (cyclic)

Horizontal range: 13,670 yards (12500 m) maximum

Slant range: 4,375 yards (4000 m) effective

Crew: 4-6

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1 Comment

  1. The L70 in the UK`s Royal Artillery was also controlled a radar aided fire control system, the FCE7, this was issued to each L70 gun detachment on a one to one L70 gun equipment. The FCE7 was mounted on the same kind of mobile gun platform as an L70 and was towed by the same kind of artillery tractor as the L70.

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