Anthony G Williams

Revised 23 October 2011

 (with thanks to those who have commented)

What exactly is "the MCG Problem"? MCG is the shorthand for Medium Calibre Gun - that is, between the light anti-aircraft cannon of up to 40mm calibre and the heavy guns of 6" (152mm) and upwards which have now been retired along with the last of the cruisers and battleships which carried them. The current weapon in the Royal Navy is of course the 4.5" (114mm) Mk.8, with other nations selecting guns of from 57mm to 130mm calibre. So why is it a problem? Because navies around the world can't agree about what the MCG is for, and therefore what kind of weapon is needed.

There have always been arguments about MCGs. During the Second World War, before missiles were even a distant dream (or nightmare), guns ruled. A wide variety of MCGs were used, the principal calibres in the RN being 3", 4", 4.5", 4.7" and 5.25" (76-133mm). The reason for such a diversity was the constant debate between the advantages of shell weight, for anti-ship purposes, and rate of fire, for dealing with aircraft.

By the end of the War, the proven danger from enemy aircraft had led to most MCGs being fitted in high-angle mountings to produce DP (dual purpose) weapons, capable of both surface and AA fire.

Postwar Developments

After the Second World War the Allies had time to reflect on the lessons. It had become painfully apparent that MCGs had to be able to put up a high volume of fire against aircraft. This led to much activity in developing automatic loading mechanisms to speed up the rate of fire, a phase which faded after the 1950s (except for smaller calibres) because of the expectation that guided missiles would take over the AA role.

Furthermore, the new proximity fuzes meant that the old concept of barrage fire - putting up a curtain of high explosive in the path of attacking aircraft - was no longer appropriate. Shooting had to be sufficiently accurate to ensure very near misses, and that meant a high muzzle velocity to minimise the time of flight of the shell. The combination of high rate of fire and high muzzle velocity was technically difficult to achieve and took years of development to perfect. In the meantime, the Royal Navy relied on the semi-automatic twin 4.5" Mk.VI DP as the standard armament for postwar frigates and destroyers. This was capable of a rate of fire of 20 rounds per minute (rpm) per gun, nearly double that of earlier mountings, but still only half that of contemporary fully automatic guns such as the American single 5" (127mm) Mark 42 and the Swedish twin 120mm.

Despite the undoubted success of the 4.5" Mk.VI mounting, it has to be said that its longevity was due more to accident than design. During the 1950s the Navy planned several fully-automatic weapons which saw little or no service use. Various 5" (127mm) calibre designs were considered, with rates of fire up to 60 rounds per minute. A 4" (102mm) Vickers gun, with a 45 rpm rate of fire, was built but sold only to Chile. The twin 3" (76mm) Mk.6, designed for up to 120 rpm (although 90 rpm was more usual), was adopted (at the time, the minimum calibre worth fitting with a proximity fuze), but only as the secondary armament of the three Tiger class cruisers, which also carried automatic 6" guns capable of 20 rpm. The USN developed a similar 3" weapon using the same ammunition, but this was unsuccessful and had a very short life.

Current Weapons

During the 1960s two completely different schools of thought developed among the world's navies. In one corner were the Americans and the British, who believed that missiles or carrier aircraft would be the primary armament in dealing with both aircraft and enemy warships. This meant that the MCG would mainly be used for shore bombardment with a backup role in dealing with smaller ship targets not worth a missile. In fact both navies went through a period when they assumed that certain classes of warships did not need an MCG at all - which in the Royal Navy led to the first two batches of Type 22 frigates - but soon learned the error of their ways when the Falklands conflict re-emphasised the importance of naval gunfire. The third batch of the Type 22 acquired a 4.5" gun.

It should be noted in passing that the US Marines have always been strong proponents of shore bombardment in order to support opposed landings. Their influence had much to do with the resuscitation of the 16" (406mm) gunned battleships in the l980s and also led to other experiments with 8" (203mm) and 155mm naval guns. Other navies, including most of those in Western Europe, decided that the gun still had an important general purpose role and would need to deal with targets such as fast missile boats and even anti-ship missiles as well as aircraft.

These philosophies led to different approaches in gun design. The British and American requirement did not call for a high rate of fire, so their mountings, the 4.5" Mk.8 and the 5" Mark 45 respectively, achieve only 20-25 rpm but have the benefit of being simple and relatively light at around 25 tons, as well as low on manpower demands. The current Mod 4 mounting for the 5" gun has a longer (L/62 rather than L/54) barrel and a longer recoil stroke to cope with more powerful ammunition, with the rate of fire further reduced to 16-20 rpm.

The alternative approach has continued to stress rate of fire. The most recent, export version of the French Creusot-Loire 100mm Compact weighs only 14 tons (19 including the hoist and magazine) and has a rate of fire of up to 90 rpm. The Italian OTO Melara 76mm weighs just 7.5 tons, with a rate of fire in the latest "super-rapid" version of 120 rpm. The Italian gun has become something of an international standard weapon, seeing service in some three dozen navies.

Two attempts have been made to enjoy the best of both worlds. Oto Melara offer a 127mm gun, using the same ammunition as the American 5", but firing at a rate of 45 rpm. The penalty for this performance is a weight of 37 tons, although this has been reduced to 22 tons in the latest lightweight version. The Swedish firm of Bofors developed an even more spectacular weapon in the late 1960s, the 120mm single, in a 28.5 ton mounting, which fires at a still remarkable 80 rpm, but this saw very little use.

The Russians have covered all of the bases, developing modern, fast-firing automatic guns in 76mm (AK-176M: L/59 calibre, 120 rpm RoF), 100mm (AK-100: L/59, 60 rpm) and 130mm (AK-130: L/54, 84 rpm for twin mounting) calibres, but current production is focused on the 100mm A190(E). This weighs about the same as the older 76mm gun but claimed range and accuracy are doubled and lethality nearly so. Compared with the AK-100, it offers an increase in rate of fire to 80 rpm and three times the accuracy. It is also available with guided and rocket-assisted long-range shells.

Most navies have taken a fairly consistent view of their MCG requirements since the l960s. One notable exception, however, has been the Canadians. Their first postwar escorts were fitted with 3" guns; either the fast-firing high-velocity British Mk 6 (which remained in Canadian service for far longer than with the RN) or the less advanced, but lighter, American Mk 33. The Iroquois class of the early l970s saw a radical change with the adoption of the 127mm OTO Melara gun. When these ships were refitted in the late 1980s, the gun was replaced by the 76mm OTO Melara. Finally, the latest City class frigates are equipped with the 57mm Bofors gun, whose 220 rpm rate of fire makes it exceptionally effective against aircraft, missiles and fast patrol boats but of limited use in other roles.

The Current Dilemma

So which philosophy is correct; small calibre and high rate of fire or larger, slower-firing guns? The Falklands conflict provided evidence to support both viewpoints. The shore bombardment role (NGS = Naval Gunfire Support) was clearly important, with the 4.5" guns firing some 8,000 rounds. On the other hand, there was a desperate need for more close-range AA fire in the confined arena of San Carlos Water, where the 76mm OTO Melara or particularly the 57mm Bofors would have been in their element. Curiously, the 4.5" Mk.8 fire control system was reportedly optimised for AA fire and was not well designed for NGS, with four crewmen needed in the command centre to operate the weapon.

The Gulf War of 1991 was significant in many ways. The lack of air opposition clearly gave the advantage to the heavier cannon; it was the last occasion on which a battleship fired its guns in anger. On the other hand, it was also the first time that a battleship was engaged by an anti-ship missile, fired from the shore, which fortunately failed to strike its target. Finally, the most significant aspect was the massive naval bombardment - carried out almost entirely by guided missile.

So where do we go from here? Specifically, what kind of gun should the next generation of warships be equipped with? In the case of the Royal Navy it could be argued that since the Falklands, the majority of warships have been equipped with Sea Wolf, and/or an anti-missile gun such as Phalanx or Goalkeeper, so the need for the MCG to engage in AA fire has disappeared. It could also be observed that both the Gulf War and the Falklands, where HMS Glamorgan was hit by an Exocet missile, demonstrated that shore bombardment has become a hazardous operation in many parts of the World. Anti-ship missiles can be launched from mobile land-based mountings and have a longer range than current naval guns, so perhaps the conventional large-calibre gun is becoming obsolescent.

The USN is examining the use of GPS-guided rockets for shore bombardment. The use of unguided missiles for this purpose is not, of course, a new idea as barrages of rockets were used for shore bombardment in the Second World War, but the modern versions have a much longer range and even the unguided ones, such as MLRS, are far more accurate. They can also carry a wide variety of payloads including anti-tank munitions.

What does that leave for the MCG to do? This is clearly the crucial question as it determines what sort of gun should be fitted. The range of possible roles for the MCG are: backup anti-aircraft and (possibly) anti-missile fire; destruction of small naval targets such as fast patrol boats; destruction of low-value ships not worth an anti-ship missile (as in the Falklands); backup anti-ship weapon in naval engagements; and shore bombardment when no enemy anti-ship missiles are expected.

The MCG also has the advantage of providing a more measured response than missiles, which is likely to prove increasingly important in the kind of United Nations or NATO "police actions '' in which the Western navies are frequently engaged today. An anti-ship missile is an all-or-nothing weapon. A gun can fire warning shots or inflict limited damage in order to persuade a recalcitrant ship's captain of the error of his ways without going so far as to sink his ship. The same measured response can also be used in shore bombardment as was done in South Georgia, at the start of the Falklands conflict, when Argentine positions were bracketed in a display of force but deliberately not hit (the conflict had yet to become "hot").

For such limited "demonstration" actions it doesn't really matter what sort of gun is fitted, as long as there is one. At present, the 76mm OTO Melara continues to dominate in international sales but the 57mm Bofors is catching up, particularly in the USA (a previous 76mm user) where the Bofors has been selected for all three of the Coast Guard Cutter, DDG1000 and Littoral Combat Ship programmes. In Russia, the St Petersburg "Arsenal" plant has developed a new 57mm gun (designated A-220) with a rate of fire of 300 rpm, intended for patrol boats. However, there is still strong interest in larger-calibre weapons for NGS, in the immediate future focused on 127mm guns but with the prospect of even larger calibres later on. The problem is that when it comes to serious naval action, the best type of MCG to have depends entirely on the circumstances and these cannot be predicted in advance. This suggests that a general-purpose weapon, with some capability in all possible roles, would be the best choice.

The most impressive Western MCG is probably still the 120mm Bofors - one wonders what it would be capable of today if its development had been continued. Of the current weapons, the French Creusot-Loire 100mm Compact seems on paper to have a lot to offer. The 13.5 kg shell is significantly lighter than the 21 kg of the British 4.5" or the 31.7 kg of the American 5", but the 90 rpm rate of fire is four times as fast as the British weapon and the gun system is claimed to have anti-missile capability. Rather surprisingly, it seems to have been almost ignored in terms of international sales (although China appears to have copied the design), and recently the 100mm guns have even been abandoned by the French Navy, which has chosen the 76mm OTO for its new FREMM frigates.

The best contender for current warships is probably the 127mm OTO Melara, as it combines the most powerful ammunition (the American 5") with a reasonably high rate of fire. It is the closest Western weapon to the formidable Russian 130mm twin mounting. It is therefore the best choice for the anti-ship and shore bombardment roles, particularly as shore bombardment missiles, other than the extremely expensive Tomahawk, are a long way from service. Italy has selected a new lightweight version of this mounting for its next generation of warships, featuring a 64-calibre barrel but with the rate of fire reduced to 35 rpm (clearly, the requirements of land attack are now dominating). For now, the Royal Navy has adopted a modified (all-electric mounting) Mod 1 version of the 4.5" Mk 8 and developed an extended-range (27 km) base-bleed shell, although a different gun seems likely to be fitted to the next class of warship (the Type 26 frigate), with the option of retro-fitting to the Type 45. At one time the 155 TMF was favoured (see below) but this was cancelled in 2010 so following consideration of both the US 5" and Italian 127mm guns, the former was selected. The main motivation for this change is presumably to avoid the continuing costs of developing and producing new ammunition for the small number of 4.5" guns in service.

The Future

For the future, there is an alternative approach to this whole problem, based on the use of steerable, guided ammunition. The Americans have developed a laser-guided version of the 5" shell (which carries with it the problem that somebody, somewhere, has to illuminate the target with a laser) and the French reportedly considered an infra-red homing version of the 100mm shell, but it was never introduced. Other self-contained homing methods such as millimetre-wave radar may prove even more suitable. While early guided shells were steered by cumbersome flip-out fins, the American firm of Raytheon demonstrated shells as small as 40mm which can be steered in flight by means of tiny explosive charges. However, steerable canard fins now seem to be the popular choice.

For NGS, the latest version of the American 5" (127mm) gun, the Mk 45 Mod 4, has a longer (62 calibre) barrel and can also tolerate higher firing pressures. In combination, these changes were expected to enable Raytheon EX-171 steerable extended-range guided ammunition (ERGM), using GPS and INS position-fixing, to reach out to 63 nautical miles (117 km) with an error of only 20 metres. It was designed to follow a ballistic trajectory up to 70 km and a 'non-ballistic' trajectory (i.e flying...) after that. The projectile is 1.5 m long and thus needs a 'double stroke' of the loading system to ram it, thereby halving the rate of fire to 10 rpm for the first minute and only 2-4 rpm after that, as the high-energy charge overheats the barrel. The shell originally contained 72 EX-1 dual-purpose bomblets, but this was later replaced by a unitary blast fragmentation type warhead. However, it was announced in March 2008 that the project had been cancelled following a troubled development history.

Two other American projects have aimed to enhance the NGS capability of existing as well as new 127mm guns. One was BARRAGE, a fin-stabilised sub-calibre, GPS-guided round  which was capable of 74 km and had multiple-flechette warheads for use against personnel. This programme was terminated in 2002. The other was the Autonomous Naval Support Round (ANSR), later renamed the Ballistic Trajectory Extended Range Munition (BTERM), which was tested out to 100 km from a 54 calibre barrel - it used a prefragmented HE warhead derived from the HARM missile which weighed about 25 lbs (11.3 kg), including 10 lbs of HE (4.5 kg) fitted into a tungsten case which fragmented on detonation. The fragmenting warhead, coupled to a precision-guidance system, provided four times the lethality of a steel-cased warhead of equivalent size. BTERM was only a demonstration project for a system which would provide a low-cost solution, permitting volume as well as precision fires. ATK announced in a press release dated 11 May 2004 that: BTERM had "captured the highly competitive award for development of the Navy five-inch Extended Range Munition (ERM). With a value of $30 million, the contract calls for ATK to perform a 16-month demonstration of an innovative extended range, low cost, gun launched projectile capable of operation in the Mk 45 Mod 4 and Mod 2 naval gun systems." However, this was cancelled in 2007 although a requirement for an Extended Range Munition was stated still to exist. By 2011, it was reported that the USN had dropped the ERM programme with no successor currently in prospect.

In Europe, the Italians and the Dutch have developed the Vulcano, a family of HE fin-stabilised discarding-sabot (HEFSDS) 127mm rounds. This will include an unguided multi-purpose round with a range of up to 70 km (84 km from the L64 barrel), an IR-terminally guided anti-ship round with the same range, and an INS/GPS shore bombardment round with a range of up to 120 km from a 64 cal barrel (100 km from L/54). These are one-piece rounds which can be fired at 35 rpm in the new OTO-Melara lightweight gun.

The Italians are also developing the Davide anti-missile programme for the OTO 76mm gun (known as Strales in its export version). This uses a subcalibre DART (Driven Ammunition Reduced Time of flight) round fired at 1,200 m/s, rather similar in appearance to APFSDS tank gun rounds.  Effective range is expected to be 5 km with engagements possible down to 2 m above the sea surface. On the face of it there is room for some concern about the effectiveness against anti-ship missiles of what must be a small (proximity-fuzed) warhead given the very high closing speeds, but they seem confident. A 'hit-to-kill' mode would seem preferable, but as the chosen guidance method is an RF beam-rider, it is probably not accurate enough for that at longer ranges. Development of a 40 km range fire support round is also being considered for the 76mm OTO.

The latest Russian development is the 130mm A192M(E), a single barrel mounting which, compared with the AK-130, is only one-third of the weight. It has a reduced rate of fire (optimised for anti-ship and shore bombardment) and stealth characteristics. In addition, long range guided projectiles are being developed for this weapon and the AK-130.

It was discovered late in 2006 that Russia has developed a new 152mm gun, with two barrels, vertically stacked. This gun has initially been developed for an army SPG, and in this form has a 50-round automatic launcher (the turret is unmanned) and can fire at about 15-18 rpm. There was also a proposed naval version, possibly intended as a replacement for the AK-130. It was being developed by Arsenal, and used a 'stealth' cupola. Range with existing ammunition was stated to be about 50 km, but NIIP was developing a new round capable of up to 70 km. These status of these guns is unclear. Illustrations of the army gun are HERE.

Put all of these developments together and it is clearly technically feasible to design shells which can home in on their targets, be they ships, aircraft or missiles, as well as hit known fixed targets with precision. This has two implications for gun design. First, rate of fire is no longer so important as the kill probability of each shell will be many times higher. Secondly, if so much expensive electronics is going to be packaged into each shell, then the best value is obtained by making the shell as large as possible.

The Russians already seem to be moving towards the 152mm calibre (see above). For western nations, the logical calibre to choose would be l55mm, to obtain some commonality of projectiles and submunitions with the Army's artillery (a significant issue as guided projectiles, submunition carriers etc are expensive to develop and manufacture), and naval weapons of this calibre have recently been under development in both the USA and Europe. Modern long-range 155mm artillery shells can be fired to over 45 km - about the same as Exocet MM38. The new extended-range technology could more than double this engagement range; the new LRLAP (long range land attack projectile) fin-stabilised rocket-assisted guided shell being developed for use in the 155mm Advanced Gun System for the new DDG-1000 destroyers is expected to be capable of 180 km. It weighs 102 kg compared with 54 kg for the standard artillery shell. The AGS is a massive and complex system, weighing 95 tons for the turret alone (the barrel is liquid-cooled to permit a constant 12 rpm) and nearly 300 tons including a full 750-round magazine, so it will be for big ships only. Rather ironically, there are no current plans to use existing 155mm artillery shells in this gun.

A simpler alternative would appear to be to use existing turrets from army 155mm self-propelled artillery. BAe Systems proposed a weapon based on the British AS90 Braveheart SPG, intended to achieve 18 rpm. France also considered such a solution based on the turret of its GIAT 155mm/52 gun (19 tons unloaded) together with PELICAN guided ammunition, with a range of 85 km. The Germans actually mounted an 18 ton turret from their 155m PzH 2000 SPG to the F124 class frigate Hamburg for demonstration purposes (made easier by their MEKO modular armament system for warships). The concept was known as MONARC (which stands for the German for Modular Naval Artillery Concept for Naval Gun Fire), and is claimed to be capable of 10 rpm. However, the difficulties of adapting the turret for naval purposes led to the cancellation of the project in 2007, and a decision to buy the 127mm OTO naval gun instead for the German Navy's next class of warships.

Army turrets need considerable modification to compare with purpose-designed naval guns. The MONARC installation required the turret to be installed in a flexible mounting to help absorb the recoil. It is also necessary to fit a gun stabilising system to compensate for ship movement, and the ammunition storage and handling systems need to be modified. Perhaps the most obvious difference is that army guns do not use naval-style fixed ammunition (i.e. the cartridge case and projectile fixed together so they can be mechanically handled as one unit). This implies that either new cartridges (and thereby new guns) need to be developed, or it will still be necessary to have manual handling of modular propellant elements in the turret, something which naval guns moved away from several decades ago. The AS90 at present also has an air-cooled barrel, which means that although it can fire 10 rounds in the first minute, it can manage only 6 rpm for three minutes. BAe were presumably proposing to fit a new water-cooled barrel to achieve 18 rpm, which would account for the quoted weight of 29.5 tons (excluding magazine).

More recently BAE Systems changed tack and developed a new concept, the existing 4.5 inch Mk 8 naval mounting with the gun switched to the 155mm L/39 from the AS90 (surplus barrels being available). This was known as the 155 TMF (Third generation Maritime Fire support). The existing mounting is apparently strong enough to stand the additional weight and recoil (and could also accept the 155mm L/52 if required). The weight of the 155 TMF mounting went up from 22.5 tons (Mk 8 Mod 1) to 24.5 tons. although this is still lighter than the original 4.5 inch Mark 8 Mod 0 at 26.4 tons. Other modifications needed to the mounting include a double-stroke loading cycle to fire the separated ammunition (which would presumably halve the RoF to around 12 rpm) plus some adjustments to accommodate the wider ammunition. It appeared that the gun would use a single-module L10 artillery charge. Obvious advantages include commonality of gun and ammunition with the British Army (with a huge long-term saving in future ammunition development costs), 80% commonality with the existing Mk 8 Mod 1 mounting without requiring the "navalisation" of an army turret, and greater destructive power than the 5 inch gun with a longer range than even the new 4.5 inch Extended Range ammunition: 30 v. 27 km. There was clearly the potential for far greater range and effectiveness increases in the future using advanced ammunition, including guided projectiles; for instance, Italy is planning a 155mm artillery version of the 5 inch Vulcano ammunition (see above). The RN was reportedly very keen on the 155 TMF project, resulting in the award of government development funding in 2007. The main problem to be solved was the handling of the propellant charges, which the RN requires to be encased for fire safety reasons. Sadly, this very promising and relatively low-cost project was a victim of the swingeing cuts in defence projects in 2010.

All of these developments are or were intended to improve the range and accuracy of gunfire support, moving the warships away from the risk of counterattack from land and/or allowing them to extend their fire support far inland. However, there is clearly the potential to develop different kinds of ammunition for other purposes. A destroyer armed with such a gun and homing ammunition, capable of engaging aircraft and possibly missiles as well as ships and shore targets, would have a powerful and versatile back-up to its long-range missiles. A frigate so armed would arguably not need missiles at all.

Future generations of frigates might look very different, with a 155mm gun backed up by a CIWS such as the 30mm Goalkeeper or the new 35mm Oerlikon Millennium. Time will tell!