In focus: the Bofors 57mm Mk 3 gun that will equip the Type 31 frigates

The Type 31 frigates will introduce two guns types new to Royal Navy service. In the second of two articles examining these weapons, we look at the Bofors 57mm Mk3 gun.

The RN has not had a 57mm gun in service since the 1950s. There were two separate “6-pounders” designs of this size developed for naval use during the second world war. Guns of this approximate calibre were seen as having a good balance between hitting power while being light enough to fit to small combatants or to larger ships without adding excessive top weight.

The Quick-Firing (QF) 6-pounder 10 cwt (47 calibre) gun was a twin-barrelled weapon originally developed for coastal defence. It had a range of about 10km range and could fire up to eighteen 2.97kg shells per minute. It was subsequently fitted to a handful of destroyers operating in the North Sea for defence against motor torpedo and motor gunboats. An entirely different weapon was the 6-pdr / 7cwt QF Mark IIA (43 calibre) with a Molins autoloader, developed by the Army for equipping tanks. It was adopted by the RN for coastal forces and fitted to many MTB/MGBs in the power-operated Mark VII mounting. It was highly effective in the role, firing up to forty 2.7kg shells per minute. The gun was also fitted to 30 RAF Coastal Command Mosquito FB Mk XVIIIs, (informally known as the ‘Tsetse’) equipped with armour piercing rounds to penetrate the pressure hulls of surfaced U-boats.

Development of the Bofors 57mm of today began in the early 1960s. This 70 calibre weapon was developed in Sweden as the SjöAutomatKanon (SAK) L/70 based on the 57mm SAK L/60 built in the post-war period for the Swedish, French and Dutch navies. The L/70 Mk1 first went into service in 1964, primarily in the anti-aircraft role with an increased rate of fire, water-cooled barrels and improved elevation and training speed over its L/60 predecessor. The Mk2 arrived in 1981 with a lighter mount and improved servos for better accuracy. Bofors claimed it was good enough for use against sea-skimming missiles but export sales were rather limited and its performance compared poorly with the highly successful OTO-Melara 76mm that has dominated the market for medium calibre naval guns.

Despite the lack of interest in the Mk2, Bofors (taken over by United Defence and then BAE Systems) continued to invest in developing the 57mm, focussing on improved accuracy, a lighter mount with low radar cross-section and most importantly, smart ammunition capability. The Mk3 first went to sea in 1998 aboard the Finish Fast attack craft FNS Hamina. The weapon has subsequently become a major export success, its adoption by the Royal Navy is the latest achievement.

The US Coast Guard selected the 57mm for its Legend-class National Security Cutters and was followed by orders to equip both variants of the USN’s Littoral Combat ship and the new Constellation-class frigates (FFGX). The USN designates the weapon as the Mk 110 Mod 0 57mmm, but BAES markets it simply as the Bofors 57 Mk3. The navies of Brunei, Canada, Finland, Germany, Indonesia, Malaysia, Mexico, Norway and Sweden are also customers. The worldwide user base, now including 5 NATO nations, improves logistics commonality and helps reduce development and production costs. Besides the manufacturer’s trials, the RN can also derive great confidence in the weapon from the extensive pre-qualifying and testing done by the USN which involved firing over 2,000 rounds at ranges on land and sea.

The Mk3 uses the same gun and mounting arrangements as the Mk2 but improves accuracy through smart ammunition and the addition of a small on-barrel radar that measures muzzle velocity of the departing shells intended primarily to control the fusing for the 3P ammunition when in timed or proximity airburst mode. The dual-hoist and dual ready-use system allows instant switching between different types of ammunition, although rounds must be removed manually in the rare instance of a misfire.

As would be expected from a weapon with an anti-aircraft heritage, the gun has good elevation from -10º to +77º. It has an advertised maximum range of 17km, effective to about 12 km. Against aircraft, it can engage targets up to about 7,600 m (25,000 feet) in proximity fuze mode. When the P3 shells detonate, their 2,400 pre-fragmented titanium pellets create a lethal area of about 400 square metres. The Mk3 can elevate rapidly at 44º per second and train horizontally at 57º per second. It has a variable rate of fire but the maximum is 4 rounds per second or 220 rounds per minute with a barrel life of approximately 5,300 rounds.

Despite its smaller calibre, the 57mm Mk 3s’ high rate of fire can deliver a greater weight of explosive onto a target than the ubiquitous OTO Melara 76mm. As a crude comparison, a sustained 10-second burst would deliver 16.5 kg of explosives compared with 13.75 kg delivered by the Super Rapid 76mm. The mount is considered very reliable and easy to maintain by the US Navy with a typical repair averaging 30 minutes. The gun is normally remotely controlled from the ops room/CIC by a single operator, but as a backup, it can also be locally controlled from equipment placed anywhere on the ship.

Including the two 20-round cassettes, and four 20-round ready use and intermediate magazines, a total of 120 rounds can be held ready on the mount itself. Without ammunition, the mount weighs 7 tonnes and can be fitted to vessels as small as 150 tonnes. The mount can be installed without ammunition hoists and in a non-deck penetrating arrangement that requires rounds to be manually loaded through the rear door. It must be assumed that the spacious Type 31 will have a magazine below the mount with a capacity of up to 1,000 rounds. The total system weight including a full magazine is around 14 tonnes.

What really separates the Mk3 from its predecessors is the range of smart ammunition types available. Besides training rounds, the ‘standard’ ammunition for the 57mm are 3P (Pre-fragmented, Programmable, Proximity- fused) rounds. (More detail in the previous article about 40mm). Essentially one of 6 modes can be selected a split second before the round is fired. Airburst modes cover the target with a cloud of lethal tungsten pellets designed to cripple aircraft, missiles or small boats. Timed or proximity fuses can be selected to explode the round at the appropriate moment. Armour-piercing or contact fusing can be chosen for heavier targets.

3P rounds are not cheap (approx £3,800 each) but their sophistication should require fewer to destroy each target and allows the gun to rapidly engage other threats. There are significant engineering challenges in developing gun-fired smart munitions. The 57mm 3P shell experiences forces of 60,000 G as it accelerates from 0 to 3,500mph in the length of the barrel. Despite the advances of the 3P technology, there are still limitations, particularly when engaging small boats. The air burst would be lethal to unprotected crew but the shell may lack enough explosive power to stop all but the smallest boats. Together with the need to engage ever faster missile targets, this has driven the development of three other guided munition types that are available for the 57mm.

In 2015 BAE Systems revealed it had developed the Ordnance for Rapid Kill of Attack Craft (ORKA) Mk295 Mod 1 57 mm round. Using a semi-active imaging seeker that can be laser-guided or can find its target autonomously using an image of the target uploaded just prior to firing. ORKA is steered by four folding canards and retains the same 3P fusing options but has reduced effective range of out to 10km. ORKA claims a one-shot, one-kill capability and in theory, will burst closer or using contact fuses, right on the target making it more lethal in a boat swarm attack scenario.

Presumably on cost grounds, the US Navy and US Coast Guard did not select ORKA for its 57mm and in 2018 placed a $23M order for the MK 332 Mod 0 High-Explosive, 4-Bolt Guided (HE-4G) projectile. This round was developed under L3 Mustang’s Advanced Low-Cost Munitions Ordnance (ALaMO) program. Precise details of the guidance methods are sketchy. The HE-4G uses an RF seeker to track the target and at a pre-determined time begins a guidance manoeuvre to optimise fusing time and is designed to detonate inside the target.

The most advanced guided 57mm munition has been developed by the US Defence Advanced Research Projects Agency (DARPA). The Multi-Azimuth Defence Fast Intercept Round Engagement System (MAD-FIRES) also has a rocket motor to extend range and speed. MAD-FIRES is predominately designed to counter the modern missile threat and guides itself onto the target. The seeker can continuously track and engage very fast approaching targets and re-engage any targets that survive the initial engagement. MAD-FIRES will be significantly more expensive than 3P or ALaMO rounds but much cheaper than a Sea Ceptor missile.

Fired in large numbers directly down the bearing of the target can also save precious seconds compared with expensive vertically launched missiles carried in small numbers. If MAD-FIRES works as advertised, the surface combatant may finally have a weapon capable of defence against large salvoes of hypersonic missiles that have penetrated area defences. A Mach-5 missile covers the 17km normal maximum range of the 57mm in around 8 seconds but the gun could have fired up to 30 rounds in that time.

The 57mm and two 40mm guns of that will be carried by the Type 31 make a lot of sense when countering small boat swarms, suicide boats, missiles, aircraft and UAVs. The days of medium calibre guns being used against other ships are long gone and Naval Gun Fire Support is the only other likely use. The 57mm is rather lightweight in the NGS role and operating 17km off a hostile coastline in a predictable pattern is an increasingly risky occupation. The 127mm Mk 45 Mod 4 that will equip the Type 26 frigate is a much heavier and more expensive weapon but delivers greater weight of fire at a range of up to 37km (depending on ammunition type). There is a coherent argument that says it would make more sense for the Type 26, which will spend most of its time on ASW duty and protecting the carrier to have the 57mm. Type 31, spending more time operating in the littorals, would be better equipped with the 127mm. However, the low-budget, fixed-price procurement of the Type 31 precludes this opportunity.

Thales have some previous experience integrating the 57mm with their TACTICOS Combat Management System but the Type 31 configuration will be the first time it has been combined with the 40mm Mk4 and the NS110 radar. There are some interesting challenges around which gun type (or missile) is allocated to targets and the selection of appropriate rounds. Part of the gun’s role is to replace automated stand-alone CIWS mounts so the level of automatic threat response or direct operator control for a range of scenarios and rules of engagement needs careful management.

It is too soon to say which advanced 57mm rounds might be selected by the RN but the new munitions can significantly raise the performance of what might at first appear to be a rather underwhelming weapon. To give the Type 31 significant offensive capability against peer adversaries will obviously require additional firepower beyond what has been contracted for so far but the 57mm is good value for money. If the RN adopts MAD-FIRES, the Type 31s can extend its depth of capability as an air defence escort beyond the 12 or 24 Sea Ceptors it will carry.