The Bofors Gun – Part I: the L/60 Weapons

by Chris Chant on 26/11/2012

Developed from 1928 as what was a smaller version of a 57 mm (6 pdr) semi-automatic gun developed as an anti-torpedo boat weapon late in the 19th century by Finspång. The first test weapon was in essence a re-barrelled Nordenfelt version of the Finspång gun, to which was added a semi-automatic loading mechanism.

Testing of this gun in 1929 demonstrated that there was a problem in keeping the gun supplied with ammunition to ensure a reasonable rate of fire: the mechanism was strong enough to handle the stresses of moving the large round, but was too heavy to move quickly enough to allow the maintenance of a high rate of fire. One attempt to solve this problem used zinc shell cases that burned when fired, but this left heavy zinc deposits in the barrel, and had therefore to be abandoned. In the summer of 1930 experiments were started with a new test gun that did away with controlled feed and instead flicked the spent casing out the rear, after which a second mechanism reloaded the gun by ‘throwing’ a new round from the magazine into the open breech. This seemed to be the required solution, and work on a prototype soon began.

During this period Krupp, the German armaments giant, bought a one-third share of Bofors. Krupp engineers started the process of updating the Bofors factories to use modern equipment and metallurgy, but the 40 mm project was kept secret.

The prototype was completed and fired in November 1931, and by the middle of the month it was firing bursts of two and three rounds. Changes to the feed mechanism were all that remained, and by the end of the year the prototype was firing at 130 rounds per minute. A period of steady development followed as the weapon was readied for production, this being completed in October 1933. Since the weapon had passed its acceptance trials in the previous year, this production type became the Bofors 40 mm akan m/32, which entered service in 1936.

Despite the fact that it is generally known as an L/60 weapon, the m/32 and its immediate relatives are in fact L/56.25 as the barrel is 56.25 calibres long.

The gun fired a 31.75 oz (900 g) HE 40 × 311R (rimmed) shell at 2,787 ft (850 m) per second. The nominal rate of fire was in the order of 120 rounds per minute, increased slightly when the barrels were closer to the horizontal as gravity assisted the feeding from the top-mounted magazine. In practice the rate of fire were nearer 80 to 100 rpm as the rounds were fed into the breech from four-round clips that had to be replaced by hand. The maximum attainable ceiling was 23,625 ft (7200 m), but the practical maximum was about 12,470 ft (3800 m).

The gun was provided with an advanced sighting system: the trainer and layer each had a reflector sight, while a third crew member standing behind them adjusted for lead using a simple mechanical computer. Power for the sights was supplied by a 6-volt battery.

A single mounting Bofors 40mm gunGeneric name
The weapon is now better known as the Bofors 40L60, and this manually operated equipment is still in widespread service as it uses a powerful round and can be outfitted with any of a number of simple optical sighting systems still effective in regions where less advanced attack aircraft are employed. It is found on three carriages with different weights and overall dimensions: the m/38 for a weight of 4,740 lb (2150 kg); the m/39 as described below in the specification; and the m/49e for a weight of 4,519 lb (2050 kg). In all cases the carriage is a two-axle trailer towed by any suitable 6 x 6 2.5-ton truck, and in firing position the carriage is stabilised, with its wheels lifted off the ground or removed, by an arrangement of four jacks: one each at the front and rear of the trailer, and one on each of the two lateral outriggers.

The mounting is manually powered in traverse and elevation by the laying numbers located to left and right of the breech mechanism, and each is provided with a simple sight. The ordnance is fed vertically from an overhead hopper loaded from four-round clips, the spent cases being ejected from the automatic breech down a chute under the gun. The weapon fires AP or HE projectiles at a muzzle velocity of 2,788 ft (850 m) per second. The AP projectile weighs 31.39 oz (890 g) and can penetrate 55 mm (2.17 in) of armour at 545 yards (500 m) declining to 42 mm (1.65 in) at 985 yards (900 m), while the HE projectile weighs 33.69 oz (955 g). Another major manufacturer of ammunition for the 40L60 is SNIA BPD in Italy, this company’s product range including HE-T, HEI-T and practice rounds. These rounds each weigh 4.70 lb (2.13 kg), and the projectiles of the two operational types are filled with TNT.

World-wide service
The m/36 proved immensely popular in the period immediately before World War II, and large numbers of the type were licence-built in countries as diverse as Austria, Belgium, Brazil, Finland, France, Greece, Hungary, Italy, Norway and Poland. The type was also widely produced during World War II by the British and Americans. The British model was the Automatic Anti-Aircraft Gun Mk 1 with a different carriage, including tubular rather than box-section outriggers and other features to simplify production, a travelling weight of 5,045 lb (2288 kg) and a number of detail modifications. The US Army model was variously the Automatic Anti-Aircraft Guns M1 and M2 with detail differences and a different carriage, produced in several subvariants to ease production and allow higher traverse/elevation rates, a travelling weight of 5,855 lb (2656 kg) and a number of modifications, together with a range of ammunition including AP-T, HE-T, HEI-T and TP-T, all of them 17.6 in (447 mm) long. The M81 AP-T round weighs 4.58 lb (2.077 kg) and fires its projectile with a muzzle velocity of 2,860 ft (872 m) per second, the Mk 2 HE-T round weighs 4.74 lb (2.15 kg) and fires its projectile with a muzzle velocity of 2,887 ft (880 m) per second, the Mk2 HEI-T round weighs 4.74 lb (2.15 kg) and fires its projectile with a muzzle velocity of 2,887 ft (880 m) per second, and the M91 TP-T round weighs 4.72 lb (2.14 kg) and fires its projectile with a muzzle velocity of 2,860 ft (872 m) per second. The projectiles fired by the Mk 2 HE-T and HEI-T rounds each contain 2.17 oz (60 g) of Tetryl or TNT explosive. Most of the ammunition is carried in boxes along the tops of the track guards, and is fed in four-round clips into the guides on top of each gun. The US Navy version was the Automatic Anti-Aircraft Gun Mk 1.

Improvement packages
Given the widespread nature of the 40L60’s continued service, it is hardly surprising that improvement packages have been developed in an attempt to increase the capabilities of the mounting against modern targets. One package comes from the manufacturer in collaboration with J. L. Weibull and the LIAB ammunition company. This package offers power laying, using DC electric motors powered by an on-carriage generator and driving toothed belts; an improved Bofors Aerotronics Type U optical sight fitted on a table allowing lateral and vertical movement; a Racal I/J-band ranging radar or an Ericsson laser rangefinder which supplement the optical system in feeding data to a computer which generates the correct aim-off point that is then supplied to the sight; a modernized weapon; electrical rather than manual firing; and a choice of proximity-fused PFHE or impact-fused APHC-T ammunition. Trials have shown that in action against typical targets 30% of projectiles pass within 13 ft 1.5 in (4 m) of the target and 80% within 26 ft 3 in (8 m).

As impressive in a slightly different way, however, is the package offered by an international consortium of Belgian, French and Italian companies. The addition of this package to the 40L60 produces what can conveniently be described as the Bofors 40L60 GATHER. This is in essence the 40L60 fitted with a petrol engine for electrical and hydraulic power, firing a new type of ammunition developed by Fabrique Nationale (fitted with the Thomson-CSF EMMA proximity fuse) and aimed with an Officine Galileo P56/40 sight. The package is comparatively cheap, especially in comparison with the cost of new 35 mm or 40 mm weapons which 40L60 operators would otherwise have to buy, and provides a comparatively high probability of scoring a hit despite the weapon’s low rate of fire. The newly-powered mounting is controlled by the gunner, who is located to the right of the weapon and uses a small control column. The mounting can be traversed at a maximum rate of 70° per second, with an acceleration of 95° per second², and can be elevated at a maximum rate of 55° per second, with an acceleration of 80° per second². These are calculated to provide sufficient tracking and elevation rates to allow the mounting to tackle the high subsonic targets to which the sight system is matched. FN and Thomson-CSF had already collaborated to produce the FN 128 pre-fragmented proximity-fused round for the later 40L70 weapon, and this experience stood the companies in good stead when they developed the FN 108 round matched to the 40L60. The 31.75 oz (900 g) projectile is of the pre-fragmented HE type, using a 3.175 oz (90 g) bursting charge of HMX-based explosive inside a casing that contains tungsten alloy pellets and shatters into some 2,500 fragments, of which the tungsten pellets move at a velocity of about 4,265 ft (1300 m) per second. The associated EMMA fuse has an activation range of 9 ft 10 in (3 m) and is sensitive enough to respond to missiles as small as 15.75 in (400 mm) in diameter while rejecting background reflections even when the target is as low as 16 ft 5 in (5 m). The fuse self-destructs the projectile between 8 and 10 seconds after firing, has an impact capability, and can also be retrofitted to existing HE and HEI ammunition. FN also produces an FN 109 PF/Spotter training round.

Radar-aided fire control
The P56/40 is a variant of the standard P56 computerised sight system, which weighs some 551 lb (250 kg), and requires the carriage to be strengthened despite the fact that its incorporation allows the ‘pointing’ number on the left of the gun to be omitted. The gunner has merely to track the target, keeping the crosshairs of his sight (x 5 magnification and 12° field of vision) on the target’s nose, press down the control column to activate the analogue computer, and then depress the firing pedal when the computer has solved the fire-control problem and fed the necessary lead angles into the optics through which the gunner is still tracking the target. The system can also be upgraded by use of data from an off-carriage fire-control system, the manufacturers advocating the Sanders TPQ-32 Forward-Area Alerting Radar in conjunction with the GSQ-137 TADDS (Target Alert Data Display System). FAAR can feed information to a maximum of 12 TADDS, allowing a single radar to control 12 40L60 GATHER mountings.

Naval mountings
The L/60 version of the 40 mm Bofors gun was also widely adopted for naval service on a number of mountings optimised on a national basis for particular requirements and types of vessel. The baseline model was the single mounting produced by Bofors, suitable for use on all sizes of vessel. This was first adopted in 1936, and allows the gun full traverse through 360°, if the parent vessel’s superstructure and the mounting’s location allows it, as well as elevation in an arc between -5° and +90°. The system has a maximum horizontal range of some 10,935 yards (10000 m) and an effective slant range of 3,280 yards (3000 m). The British Bofors Mk 7 naval single mounting, and its later Bofors Mk 9 mounting counterpart, with electric rather than hydraulic drive, both use the 40L60 and are still in widespread service. The specification of this mounting, which is controlled by a control column and aimed with a gyro sight, includes a weight of 690 lb (313 kg), traverse through 360° and elevation to a maximum of 90°. But the Mk 7 and post-World War II Mk 9 mountings, both using the Automatic Anti-Aircraft Gun Mk 1, must be considered obsolete in their original forms in face of a high-performance target. However, the type retains a useful dual-role capability for patrol craft and with this in mind the Royal Australian Navy has produced its Bofors 40L60 AN conversion. This uses new components because of the difficulty and expense of repairing older items, and is powered by a 7.5-hp (5.6-kW) electric motor allowing the gunner to use his control column for traverse and elevation at the maximum rates of 40° and 20° per second respectively but, perhaps just as importantly in the patrol craft application, at the minimum rate of 20° per minute. Overall some 20 modifications have been effected, but the optical sight has been retained on the grounds that the combination of tracer and HE provides the gunner of such craft with a cost-effective sight system.

Another naval mounting that was subsequently upgraded was the Canadian 40L60 Boffin mounting, though in this instance the 60-calibre Bofors ordnance was carried on the Mk V(C) mounting used during World War II with a pair of 20 mm Oerlikon cannon. The 40L60 Boffin equipments were later emplaced for the protection of Canadian air bases in West Germany. The 3,900-lb (1769-kg) mounting, operated via hydraulically assisted hand controls, could be traversed through 360° at the rate of 56° per second, and elevated in an arc between -3° and +66° at the rate of 34° per second. The weapon used a simple optical sight, and was fed with standard ammunition types in four-round clips.

Typical of the twin-gun installations for naval use is the British Bofors Mk 5 mounting, again fitted with the 40L60 ordnance. This has a weight of about 6,600 lb (2994 kg), and is fitted with a splinter shield for the on-mounting crew of three comprising gunlayer and two loaders. Like the Mk 7 and Mk 9 single mountings, the Mk 5 mounting has reversionary manual controls and a practical rate of about 120 rounds per minute per barrel. The US Navy equivalent to the Mk 5 is the Bofors Mk 1 twin mounting, which is still used in some parts of the world. This mounting weighs 13,200 lb (5988 kg) with two Automatic Anti-Aircraft Guns Mk 1, and while not fitted with a shield was generally used with its own Mk 63 optical director, located in a separate bandstand.close to the gun mounting. The Mk 1 mounting has a crew of between five and nine men.

Anti-kamikaze model
The Bofors Mk 2 naval quadruple mounting was the most potent of the US Navy mountings for Bofors 40 mm guns, and was essentially two Mk 1 mountings combined to throw up an enormous wall of fire, especially when multiple mountings are used with a higher-level fire-control system. The Mk 2 used four Mk 1 guns and turned the scales, according to specific model, at a weight between 23,800 and 25,500 lb (10976 and 11557 kg). It had a crew of 11 men.

The Bofors Mk 3 single mounting is the lightest member of the family, weighing 2,264 lb (1027.0 kg) and carrying one M1 gun. It is operated by four or five men.

Specification

Bofors m/32 40L60

Type: single AA gun mounting

Calibre: 40 mm

Barrel length: 56.25 calibres

Carriage: four-wheel platform with outriggers but no shield

Weight: 2400 kg (5,291 lb) in travelling and firing orders

Dimensions: length, travelling 20 ft 11.25 in (6.38 m); width, travelling 5 ft 7.675 in (1.72 m) and firing 12 ft 10.375 in (3.92 m); height, travelling 6 ft 6.75 in (2.00 m)

Traverse/elevation: 360°/-5° to +90°

Rate of fire: between 120 and 140 rounds per minute (cyclic) and 70 rounds per minute (practical)

Horizontal range: 4750 m (5,195 yards) maximum

Slant range: 2500 m (2,735 yards) effective

Crew: 4-6

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