The Type 31 frigates will introduce two guns types new to Royal Navy service. In the first of two articles examining these weapons, we look at the Bofors 40mm Mk 4 gun.
The RN has not had a 40mm gun in service since the Bofors 40mm L/60 Mark IX was phased out in the 1980s. The Bofors 40mm can trace its design back to the Second World War and its decedents are in still in service with many navies. The RN has standardised on the 30mm as its main small calibre gun but 40-mm weapons offer significantly longer ranges, increased lethal effect in anti-aircraft/missile and anti-surface engagements.
Bofors began work on the Mk4 in 2009 with the main goal being to incorporate fully digital modularised controls and reduce the size and weight of the existing Mk3 mounting to allow installation on a wider range of small patrol craft. Any reduction in top weight is also beneficial when selected for larger combatants. Improvements in accuracy and weight reduction has seen the 35 and 40mm naval gun sector become more crowded with weapons derived from armoured fighting vehicle guns. Offerings from other European manufacturers include the Thales/Nexter Rapid Fire CTA 40 (France), Lenardo Marlin 40 (Italy), Rheinmetall Millenium GDM-008 (Germany) and the Aselsan Gökdeniz (Turkey). The CTA 40 has been ordered for the British Army’s Warrior/Boxer and Ajax AFVs and some would see a missed opportunity for commonality across UK defence. Others might argue the RN is best avoiding any involvement with the Army’s catastrophic vehicle procurement projects. The Bofors Mk4 mount is cheaper than the CTA 40 and its notoriously expensive ammunition, likely the decisive factor for the budget-conscious Type 31.
BAE Systems (who now own the trusted Bofors brand) conducted 4 rounds of land-based testing of the Bofors 40mm Mk 4 prototype in 2011. For sea trials, the first mounting was fitted to the Swedish patrol boat HSwMS Jagaren which conducted live firings off Gothenburg in October 2012, initially firing about 60 rounds focussed on testing the feed mechanism and gyro stabilisation. The trials were a success and BAE Systems have been offering the Mk4 to navies worldwide. To date, around 50 mounts have been sold. In October 2020 the order for 10 mounts to equip the 5 Type 31 frigates was confirmed, making the Royal Navy the fifth operator in addition to the navies of Brazil, Finland, Belgium and the Netherlands. The contract with BAES/Bofors includes tools, spares, documentation and support with deliveries to the shipyard in Rosyth expected in 2023 and 2024. Although now owned by a British parent company, the gun and mounting are manufactured in Karlskoga, Sweden.
The compact all-rounder
The Mk4 mounting is more than 40% lighter, smaller and cheaper than the Mk3 with a total weight of just 2,500 kg (including 100 rounds of ammunition). Although the L/70 gun remains much the same, the mounting design is entirely new. The mount is encased in a lightweight carbon fibre dome designed to minimise radar cross-section. Heavy and more fire-prone hydraulic systems have been replaced with electric and pneumatic drives. The support ring of the gun has been increased from 900mm on the Mk3 to 1,850 mm which distributes the recoil forces over a larger bearing surface.
As would be expected from a weapon with an anti-aircraft heritage, the gun has good elevation from -20º to +80º. It has a maximum range of 12,500m, effective to about 10,000m. It is only sufficiently accurate in the CIWS role to engage missile targets at around 2.5km. It has a variable rate of fire but the maximum is 300 rounds/min with a barrel life of 5,000 rounds.
The modular digital architecture allows for flexible control arrangements. The gun is intended to be operated remotely but has gyro-stabilised local control as a backup. It can be guided by radar, EO camera or manual inputs and integrated into combat management systems. In the case of the Type 31 frigate, the Mk4s will be integrated into the TACTICOS CMS for control and be guided by inputs from the Mirador EO cameras and the NS100 primary radar.
The Type 31 will not be fitted with a ‘traditional’ CIWS such as Phalanx but the Mk4 is very capable of this role. The Mk4 in an entirely different concept and is not a self-contained bolt-on weapon like Phalanx but is integrated into the sensor system of the warships for use against a wider variety of targets than a dedicated CIWS. Phalanx will operate fully automated 24/7 using its own sensors, searching for incoming threats and responding without operator intervention if required. The 40mm mounts provides greater flexibility and can rapidly be changed to intercept different types of targets. To defend against missiles, rather than simply spraying a wall of solid 20mm tungsten rounds, the Mk4 is more accurate, has a longer reach but a much lower rate of fire and takes advantage of modern developments in ammunition.
The Mk4 was designed from the outset to use programmable 3P (Pre-fragmented, Programmable, Proximity) ammunition developed by Bofors. The gun can also fire standard and cheaper ‘dumb’ PFHE (Pre Fragmented High Explosive) or armour piercing rounds but theoretically, 3P is more efficient as fewer rounds should be needed to neutralise each target. The RN has yet to confirm if it has ordered 3P ammunition which is likely in a separate future budget line to the Type 31 ship-board equipment.
The 3P round is powered by a small internal battery which is activated by the shell entering the breech. The sensors in the 3P round are almost entirely resistant to ECM jamming and sea/ground clutter. Before each 3P shell is fired, it can be individually programmed by induction in one of 6 modes selected by the operator. This ability to switch rapidly between modes, to match different targets is a major advantage over conventional rounds that have to either be changed slowly by hand or use multiple ammunition feeds. While the gun is still in the chamber and milliseconds before firing the range (if available) can also be fed into the round to give the precise time to target.
The 6 modes are:
Gated Proximity Mode is the default and is very effective against small aircraft, UAVs and missiles, detonating at a pre-set minimum distance from the ship when sensing the presence of the targets within a narrowly defined area surrounding the shell (the round self destructs anyway after 15 seconds).
Gated Proximity Mode with Impact Priority – used against larger aircraft or helicopters intended to penetrate further into the interior before detonation.
Impact mode – explodes on first contact with a target – typically for use against small craft or lightly protected vehicles.
Armour-piercing mode – for use against larger warships, buildings or armoured vehicles. Penetrates the interior before detonation.
Timed mode – The round is set to airburst at a set distance – for use against targets ashore such as hovering helicopters, armour or troops that are hiding behind natural features. Alternatively, a swarm of small boats could be destroyed at a set range.
Mk4 and the Type 31 frigate
Some may be concerned about the loss of dedicated CIWS and just how the Type 31 will be configured to respond rapidly to missile attack is yet to be seen but with two 40mm mounts and a 57mm gun there are a lot of options. On detonation, each pre-fragmented 40mm round creates a 140 square metre lethal area of pellets and shrapnel (the 57mm gun which also has greater range creates a 400 square metre area). Combined, the 3 guns can rapidly put up a devastating wall of metal around the ship which will be hard for any missile, aircraft or boat to penetrate. To combat the massed boat or UAV swarm this is a particularly effective combination of weapons.
The 40mm Mk4 has about double the range of the 30mm ASCG DS30M Mk2 (the standard cannon fitted to the Type 23), greater hitting power, accuracy and range of munitions. The high centreline Mk4 mountings are well positioned on the Type 31 frigate with wide overlapping firing arcs in contrast to the port and starboard 30mm cannons on the Type 23s. Topweight is not a big issue for the large Type 31 but the Mk4 is 60% lighter than a Phalanx mount.
There is some controversy around the number of new gun systems that the RN is introducing but the Bofors Mk4 appears to good value, being relatively cheap and effective. Assuming the gun proves successful on Type 31, this compact and light weapon could be fitted to other RN platforms. In a future article, we will look at the Mk4’s big brother – the 57mm Mk3.