The 30mm Automated Small Calibre Gun is carried by the majority of vessels of the RN surface fleet. Here we look in detail at this ubiquitous weapon system.
The naval anti-aircraft cannon that can fire explosive shells can trace its roots back to WWII. Swiss-designed Oerlikon 20mm and Swedish-designed Bofors 40mm mounts were used throughout the war and were still in RN service with minimal modifications in the 1980s. The 1982 Falklands war demonstrated the RN’s urgent need for more capable air defence weaponry.
The BMARC (British Manufacturing and Research Corporation) offered its GCM-AO3 twin mount to the RN in 1973 as a candidate for equipping the Type 42 air defence destroyers. The tight budget, a fixation with guided missiles and top weight limitations meant no orders were placed, but in late 1982 the design was still available and went into immediate production.
The GCM-AO3-2 provided stabilised rate-aided gyro sight for the gunner, which much improves accuracy and the twin barrels delivered a combined 1,300 rounds per minute. The mounts were hastily fitted to the Type 42 destroyers and the LPDs until sufficient Phalanx CIWS mounts were available. Subsequently, the Bach I and II Type 22 frigates also received them, but as they were equipped with the Sea Wolf point defence missile, the urgency was not as great.
To replace the rather motley collection of Bofors 40mm, Oerlikon 20mm and the GCM-AO3 with their differing ammunition, training and logistic requirements, the RN opted for modern single 30mm cannon. The LS-30B mount was designed by Laurence Scott Ltd and entered service with the RN in 1989 carrying the Oerlikon KCB 30mm cannon, the system was designated in service as the DS-30B (Laurence Scott having been taken over by MSI Defence Systems Ltd). This mounting and it’s modified descendent, the DS-30M Mk II remain in service today.
The mount has a modular central yoke which and it can be configured for use with cannons between 25-40mm. It is a highly flexible design that can be customised with different ammunition arrangements, gunner’s cabin, sights, sensors or co-mounted weapons such as small missiles or other light guns. MSI-DSL markets the mount as the ‘Seahawk DS’ family but this designation is not officially used by the RN which instead refers to the whole system as the Automated Small Calibre Gun (ASCG) or more commonly, “the 30mm”.
The mount is gyro-stabilised, electrically operated and self-contained with a choice of control modes and sights. It is designed to have a low magnetic, radar and infrared signature, be reliable and easy to maintain.
These guns can be directed and fired in three ways. When in local control, (LOCSIG) a gunner aims and fires the gun while sitting in the cabin on the right-hand side of the mount. Should the electrical supply, gyro stabilisation and heading inputs from the ship be cut off or interrupted, a backup battery and manual operation by the gunner on the mount can keep the weapon in action. Alternatively, it may be remotely controlled (REMSIG) from consoles on the bridge or in the operations room. The operator aims the weapon using laser, TV and infrared imagery from the Electro-Optical Directors mounted away from the gun on platforms high on the ship.
In many situations, directing the gun using the EOD has many advantages over the naked eye of the gunner sitting on the mount, particularly at night or in poor weather. (See previous article about EODs). In AUTSIG mode, the DS-30M can be aimed by the EOD which can autonomously track targets and provide a fire-control solution directly to the mount.
The DS-30B Mk I
The original DS-30B is still the most numerous mount in RN service and as of 2003, the RN had purchased 72 systems. They are equipped with the Oerlikon KCB 30mm cannon, a design based on the Spanish Hispano-Suiza HS-831 developed in the 1960s. Oerlikon Contraves subsequently acquired the HS design and sold the weapon as the Oerlikon KCB. The KCB designation is derived from the Oerlikon naming system for guns. The K is for Kanone (gun), the second letter denotes calibre, C being 30mm and the third letter is for the gun model. (Oerlikon Contraves has subsequently been absorbed into the giant Rheinmetall Defence Group).
The KCB 30mm was conceived as an anti-aircraft weapon. It is a ‘traditional’ cannon, using propellant gas to unlock the bolt and breech during the firing cycle and can be clip-fed or belt-fed. It has a relatively high rate of fire, of up to 650 rounds per minute and an effective anti-aircraft range of about 2.75 kilometres. Firing horizontally it can reach 10km and the mount can traverse at up to 55º per second. The Mk 1 mounting is easily distinguished from the MK2 by the square barrel mount and boxed ammunition chute but is virtually identical in weight and dimensions.
Spurred by the terrorist attack on the USS Cole in 2000 and increasing awareness of vulnerability to asymmetric attacks, the RN began the littoral Defensive Anti-Surface Warfare (ASuW) upgrade programme. This included a requirement for a more modern cannon that emphasised accuracy and reliability over rate of fire. The KCB 30mm is ideal for putting lots of metal into the air to defend against high-speed aircraft but is less well suited to smaller, slower and sometimes hard to classify targets. Defence against fast attack craft and ‘swarming’ threats in the littoral areas which may include small boats, and jet-skis, perhaps armed with small missiles, rocket-propelled grenades, heavy machine guns or explosives. In the past decade, the evolving threat from UAVs and USVs now puts even greater emphasis on accuracy and automation.
The DS-30M Mk II
In 2005 the MoD awarded MSI-DSL a £15M contract for 26 DS-30M Mk II (ASCG) systems, initially for retrofitting to the 13 Type 23 frigates. Land-based trials were conducted at the Eskmeals Range in Cumbria before the first mountings were fitted to HMS Somerset in August 2007.
There are some modifications to the mount but the main difference between the Mk I and Mk II is a completely different cannon. The ATK Bushmaster II Mk 44 was developed in the US (Northrop Grumman bought out ATK in 2018) and is an exceptionally reliable chain gun. This type of weapon does not rely on the occasionally unreliable firing of the previous cartridge to cycle the action. An electric motor is used to drive a chain that moves the bolt assembly that loads, fires, extracts, and ejects the cartridges. In the event of a misfire in a conventional gun the gun has to be manually cleared which can take even a well-trained crew several minutes. A chain gun will continue to fire regardless, automatically ejecting a misfired cartridge. Theoretically, the rate of fire could be completely variable but the DS-30M has 2 settings – slow or fast and can be set to fire single rounds or continuously (maximum 200 rounds per minute).
The Bushmaster offers precise round control and is easy to maintain with low-through-life costs. Already proven in combat on Armoured Fighting Vehicles (AFV), the selection of the Bushmaster was a low-risk option. Most significantly rounds have a low dispersion rate offering better accuracy than the KCB, with an effective range of 2km and a horizontal maximum range of about 4km.
In 2016 the RN ordered another 12 Mk II mounts from MSI in a £16.5M contract that would see deliveries and support over a 5-year period. In addition to the Type 23 frigates, the 5 new batch II OPVs carry the MK II and both QEC aircraft carriers will receive 4 mounts. HMS Queen Elizabeth is due to receive her outfit in early 2021, prior to her first operational deployment. It is possible they will be REMSIG and AUTSIG-only and not have gunner’s cabins for local control.
The MK II will also be fitted to the Type 26 frigates, probably using refurbished mounts removed from Type 23s as they decommission. The Mk 1 mount is fitted to the Type 45 destroyers, the minehunters and some RFAs. Although there is space available, it seems unlikely the Type 31 frigates will be fitted with either 20 or 30mm cannons as they will have two automated Bofors 40mm Mk4 mounts.
|Vessel||DS-30B Mk 1||DS-30M Mk II||GAM-BO1|
|QEC Aircraft Carriers||4|
|Albion Class LPDs||2|
|Type 45 Destroyers||2|
|Type 23 / 26 Frigates||2|
|OPV Batch I||1|
|OPV Batch II||1|
|Sandown Class Minehunters||1|
|Hunt Class Minehunters||1|
|Echo Class Survey Vessels||2|
|Bay Class RFA||2|
|Tide Class RFA||2|
|RFA Fort Victoria||2|
|Wave Class RFA||2|
The table above shows the distribution of the 3 cannon types across the fleet and the number of mount positions per ship. It should be noted that not all vessels are fitted with these weapons at all times, particularly the RFAs. They are usually removed for refurbishment and/or use on other ships during major refit or when a ship is in low readiness.