Few would argue that medium-calibre cannons are an essential tool of modern warfare. These weapons ranging in bore diameter from 20 mm to 57 mm have been common to armoured fighting vehicles since the 1970s and provide vital fire support to infantry formations and anti-armour capabilities against their opponent’s equivalents.
Despite their long service history, many of these weapons have remained relatively unchanged with the exception of some improvements to ammunition since entering service. As a result, the Russian and Chinese armed forces still broadly rely upon the 30 mm 2A42 Shipunov cannon developed in the 1970s and its mechanically simpler descendent the 2A72, the UK’s Scimitar and Warrior are armed with the 30 mm RARDEN designed in 1966, and the US M2 Bradley retains its capable 25 mm M242 Bushmaster, which was designed in 1976. With the occasional exception of the 40 mm cannons used by the CV9040 and the K21 from South Korea, and a few countries that operate 35 mm weapons, the 30 mm cannon has broadly dominated the medium-calibre market. Nevertheless, times are changing and larger calibres are being developed to meet the perceived need for greater lethality and more complex operational profiles.
The need to modernise many of these cannons has been acknowledged since the early 1990s as increasingly well-protected light vehicles entered service such as the Russian BMP-3. Initial developments took advantage of growth possibilities within existing ammunition natures, leading to the development of the M919 armour-piercing fin-stabilised discarding sabot (APFSDS) round for the 25 mm M242 Bushmaster cannon that arms the Bradley IFV. Advances in munitions and fire control systems have led to an expansion of the role of medium-calibre cannons. Some are capable of firing air bursting munitions (ABM), which enable them to engage aerial targets and infantry in defilade. APFSDS rounds are fairly common, bringing significant lethality against lightly armoured vehicles over armour piercing discarding sabot (APDS) rounds that were prevalent following the Second World War and early-mid Cold War. APFSDS technology for medium-calibre weapons is particularly significant as the greater length to diameter ratio enabled by the fin stabilisation improves penetration of armoured targets. APDS rounds are spin stabilised, which means that they cannot exceed a certain length without becoming unstable. The 25×137 mm PMB090 APFSDS round, for example, was capable of penetrating 36 mm of rolled homogeneous armour equivalent (RHAe) set at an angle of 60 degrees from a range of 1 km – a 50% increase in penetration over the first generation of M791 APDS rounds developed for the M242.
Generally however, the technologies that are currently being explored for the modernisation of the medium-calibre weapons arming armoured fighting vehicles are ironically based on technologies that are as old as the in-service weapons, or have been investigated extensively and discarded by some. There are three identifiable trends as far as the weapons themselves are concerned: Cased telescoped weapons have cornered a small section of the market; Larger calibre conventional weapons are favoured by the major players such as Russia and the US; And some are examining the suitability of dual calibre weapons such as the 50 mm ‘Supershot’.
The US Army is investigating the utility of Northrop Grumman’s XM913 50 mm chain gun, a member of the venerable Bushmaster family. The weapon has two ammunition natures designed for it including the XM1204 High Explosive Airburst Tracer (HEAB-T) and XM1203 APFSDS-T. The XM913’s ammunition is noticeably much larger than the 25 mm rounds fired by the M242, and the barrel too is much larger providing, an effective range in excess of 3 km. However, not all is quite as it seems, the XM913 in fact leverages a cartridge known as 50 mm ‘Supershot’, which essentially takes the 35×228 mm cartridge used with the Bushmaster III, ‘necks-up’ the cartridge, widening the neck to make the overall round cylindrical rather than bottle-shaped, and increasing the diameter of the projectile correspondingly, resulting in the 50×228 mm ‘Supershot’ cartridge. The technology has been presented as a new development at times, but the concept can be traced back to the Rh503 developed by Rheinmetall as a replacement armament for the Marder IFV in 1987. The Rh503 was chambered in 35 mm, but could fire a 50 mm round that had the same base diameter and length as the 35 mm round. All that was required to fire the larger round was a new barrel, however the recoil and feed systems for the 35 mm round needed to account for the larger forces and changes in the overall dimensions of the 50 mm round, thereby increasing design complexity.
In 1997 the then McDonnell Douglas formed a consortium with Boeing, Mauser, Alliant, and Oerlikon to promote the Bushmaster III and Rh503 35/50 cannons internationally. In essence, the XM913 which is designed to provide next-generation lethality for the US Army, is a peer of the Anglo-French Cased Telescoped 40 (CT40) weapon. They were developed at similar times to address similar needs, and have both not achieved the attention that they were expected to.
The 50 mm ‘Supershot’ concept is interesting and does provide an increase in lethality. The much larger projectile – especially when using ABM or high explosive (HE) ammunition natures – and increased range dramatically increase combat reach over the 35 mm alternative. However, because the cartridge size is not the same as a ‘true’ 50 mm weapon, the projectile travels much slower and carries less energy – thereby producing a less effective weapon when compared with a ‘true’ 50 mm weapon. Brasseys indicates that the muzzle energy of a 50 mm ‘Supershot’ is somewhere between a conventional 40 mm and 45 mm weapon. This is not to say that a 50 mm ‘Supershot’ is an ineffective weapon; it is more lethal than the 35 mm cannon that it evolves from, and matches a conventional 40 mm round for muzzle velocity, at 1,600 m/s. However, it is important to place the Supershot 50 within the appropriate context. Like most of the cannons considered here, it is not an entirely new development, and there are some, such as the CT40, that seek to offer different capabilities in a smaller package.
The Revolution that Never Was
The idea of cased telescoped ammunition (CTA) has been around since the 1950s and was explored by the US Army and Air Force Combat Vehicle Armament Technology (COMVAT) and Advanced Gun Technology (AGT) efforts respectively, which ran as experimental programmes from 1973 through to the 1990s. The French also launched a CTA development that explored a 45 mm weapon, and the UK’s Royal Ordnance examined a similar weapon at around the same time. Despite this, there are only two vehicles armed with CTA weapons – the French EBRC Jaguar and the UK’s Ajax.
Cased telescoped weapons promised to revolutionise the world of medium-calibre weapons by reducing the size and weight of larger calibre weapon systems that were thought to be required to defeat evolving threats such as the BMP-3. Weapons in this category employ a tubular ammunition case that surrounds the projectile with the propellant giving the ammunition the appearance of a cylindrical drinks can, as opposed to a bottle shaped round with the projectile protruding from the neck of a cartridge. This ammunition enables a unique type of breech and ammunition handling system. The breech is typically fixed in place, allowing the chamber to rotate, bringing a round to the breech to fire, during which the previous round’s casing is ejected by the incoming fresh round.. This reduces the working parts of the system and facilitates a rate of fire of 200 rds/min in the CT40, although GIAT’s 45 mm M911 cannon could achieve 400 rds/min, due to rotating the breech through a smaller angle.
A number of arguments have been advanced in favour of CTA, such as the reduced volume required to store the ammunition compared to conventional bottle-shaped ammunition of the same calibre. For example, in the 1990s, an M2 Bradley and Warrior were reported to be respectively capable of carrying 195 and 70 rounds of CTA, as opposed to 160 and 60 rounds of conventional 40 mm ammunition. The reductions possible in ammunition length in turn have precipitated advances in breech design, which mean that a 40 mm cased telescoped weapon would only take up as much space in a turret as a 25 mm cannon, whereas a conventional 40 mm cannon requires significantly more. This enables the cannon to be elevated to very high angles, which is of undoubted value in battlefields dominated by small UAVs and urban environments.
At the same time, armour piercing lethality is retained, despite a reduction in the overall size of the round. A CT40 weapon tested by the US in 2000 fired a 250 g tungsten monobloc APFSDS penetrator (note: this weight does not include the sabot) with a pull sabot at a muzzle velocity of 1,600 m/s. The velocity decayed at a rate of 120 m/s/km. These figures suggest a muzzle energy of 0.32 MJ, close to three times that of the Russian 30 mm 2A42 cannon, and approximately equal to a 40 mm Mk 1 round fired from the BAE Systems 40 mm L/70B cannon. The round is reportedly capable of penetrating 140 mm of RHAe at 1,500 m, making it competitive with conventional munitions of the same calibre.
The cartridge design allows preformed and consolidated propellant to be used, which can be designed to fit around the projectile and lead to optimal propellant mixtures and quantities, which in turn impacts the muzzle velocity and accuracy of the ammunition, which are both critical characteristics for advanced natures such as APFSDS and ABM. The ammunition handling system (AHS) means that natures can be mixed within a single burst – for instance if dealing with infantry in a building or behind cover, a point detonating (PD) round could be used to knock a hole through a wall, and be followed by two ABM through the same hole.
Regardless of the benefits, it is apparent that CTA weapons have failed to revolutionise the medium-calibre market. Working designs have existed since the 1950s and yet the uptake is limited to the UK and France. A CTA weapon is being explored by South Korea as the future main armament of an IFV to replace the K21. It also seems that a weapon was under development in Japan in the 2000s, although it is not clear whether the system ever progressed beyond this phase. Russia’s TsNIITochMash signalled in 2019 that it was reviving work conducted in 2010 to develop a 45 mm CTA weapon, which was expected to compete with TsNII Burevestnik’s 57 mm 2A91.
S-60: Old Dog, New Tricks
The AU-220M Baikal turret promises to generate a wealth of survivability challenges for NATO states. It is armed with a fearsome 2A91 57 mm cannon against which there are few good survivability options. The 2A91 cannon is itself a modernised derivative of the legacy S-60 air defence cannon chambered in 57×347SR. The cannon is sometimes also referred to as the AZP-57, it is a rifled cannon loaded by four-round clips. The S-60 entered service in 1950 on the AZP S-60 towed anti-aircraft (AA) gun to provide the Soviet armed forces with a replacement for its legacy 37 mm air defence cannons. It served at the divisional level in batteries of batteries of six, with assistance from radar-based fire control systems. By around 1955, the weapon also entered service on the ZSU-57-2 self-propelled anti-aircraft gun (SPAAG). The weapon was eventually superseded by guided missiles and more effective means of countering air power. However, it remained in the arsenals of many countries and has found a variety of new uses.
Aside from the Russians, other have also experimented with 57 mm armaments. In the 1970s the Bofors 57 mm cannon, chambered in 57×438R, was considered as a possible main armament for the prospective Begleitpanzer 57 fire support vehicle, but this vehicle was ultimately not adopted. Elsewhere, in the 1990s Ukraine’s industry mounted an S-60 externally onto a BTR-80, however this too failed to enter service. Since the Syrian Civil War the S-60 has been mounted onto tank and truck chassis to provide direct fire against vehicle-borne IEDs and other targets. More recently the S-60 has been observed mounted onto the load beds of trucks in Ukraine to provide direct fire support.
The S-60 was capable of attaining a rate of fire of 120 rds/min, although the practical rate of fire is typically given as 70 rds/min. It was originally developed to fire – amongst others – the UBR-281 armour-piercing high-explosive (APHE) projectile, weighing 2.82 kg. At a range of 1,000 m, this round could penetrate 96 mm of rolled homogeneous armour making it a potent anti-armour weapon. Images from the Parola Tank Museum in Finland indicate that it is capable of penetrating the turret of a T-55 from certain angles. Its large projectiles including the UFB-281 high explosive fragmentation (HE-FRAG) round also weighing 2.8 kg, were thought to be capable of downing a jet aircraft with a single shot at the time they were introduced.
The 57 mm cannon has found additional applications in its modified form, the 2A91, which is fitted to the AU-220M Baikal remote weapon station developed by Russia’s TsNII Burevestnik, a subsidiary of UralVagonZavod (UVZ) as a possible main armament for Russian IFVs. A modified version of the AU-220M with greater elevation was developed for the 2S38 Derivatsiya-PVO short-range air defence system, which entered state trials in 2018. The first reference to the AU-220M appears to come from 2005, when it was reported that the system had been developed to up-arm the PT-76. At the time it was operated by two personnel and armed with a modernised form of the S-60’s main armament including a muzzle brake to reduce the recoil imparted to the vehicle. By 2015 UVZ presented a model of the AU-220M in Abu Dhabi, it was remotely operated by this stage and reportedly fitted with armour, protecting it from 30 mm rounds across the frontal arc. Development continued and in 2019 TASS reported that work was underway to investigate the feasibility of installing the AU-220M onto the entire Russian IFV fleet. If this were to be accomplished, it would extend the reach of Russian armoured vehicles considerably. The 57 mm cannon is theoretically capable of a range of 12 km, although it is realistically designed for engagements at 4 km. This is nonetheless close to double the range of the 30 mm 2A42 cannon used by most Russian IFVs at present. It fires at a rate of 80 rds/min, but would likely be constrained by its limited magazine – also 80 rounds. In 2019 it was demonstrated firing on the move whilst mounted on a BMP-3, indicating that it could greatly increase the lethality of that platform.
Furthermore, the greater lethality of the ammunition would be a considerable boost. It is theoretically possible to armour vehicles such as Puma, Boxer, and Piranha 5 against 30 mm ammunition from the front, with some capable of carrying that armour on the vehicle sides, too. However, even the legacy 57 mm ammunition for the S-60 leaves the barrel with a muzzle energy of around 1.4 MJ (a 30 mm round is considerably lower at 0.139 MJ for the armour piercing round of the 2A42), although this energy is dispersed over a relatively wide area and would not be as effective at armour penetration as an APFSDS type, it still represents a significant challenge for armoured vehicles up to main battle tanks (MBTs). Then in 2021, imagery surfaced showing an APFSDS ammunition nature for the 2A91, which would massively extend its reach and lethality compared to older natures. Although the capabilities of the new 57 mm APFSDS round are unconfirmed, a Russian study from 2017 examined the possibility of creating an APFSDS round for the 57 mm cannon. The study’s authors suggested that such a round would be capable of penetrating more than 300 mm of RHAe, which massively overmatches the protection of nearly any armoured fighting vehicle, barring the frontal arc of an MBT.
The capabilities of the 57 mm cannon are clear, the primary question is whether or not Russia will be able to bring the system into service. The level of recoil generated indicates that an advanced fire control computer would be necessary to make the system effective. Recent events have shown that Russia is more reliant upon Western microprocessors than originally thought for its more advanced military computers. It follows that if access to these components is effectively restricted (it is worth noting that this is likely a very difficult task) then Russia may not be able to develop and produce the fire control systems necessary to make the 2A91 effective. The effect of Western sanctions should not be overstated, Russia has long experience of avoiding sanctions to acquire dual use goods, but it stands to reason that weapons such as cruise missiles will take priority when competing for potentially limited resources over fire control systems for IFVs.
Casing the PLA
The PLA employs what is widely believed to be a copy of Russia’s 2A72 30 mm cannon. The weapon arms the PLA’s ZBD-04 and ZBD-03 tracked IFVs, as well as the ZBL-09 wheeled IFV. There is no immediate sign that this weapon is considered insufficient by the PLA, although this may be a result of the organisation’s secretive approach to weapon development. However, in 2020 a video was released by a state-endorsed news channel showing a 40 mm cased telescoped weapon carried in a remotely operated turret. It may have been a further development of an earlier system presented by NORINCO in 2016, which was reportedly capable of firing CS/BAA3 APFSDS ammunition that could penetrate 130 mm of RHAe at 1,000 m and the CS/BTE3 high explosive round, at a maximum cyclic rate of fire of 200 rds/min. The weapon enabled an elevation of 87 degrees, demonstrating the high elevation range characteristic often associated with cased telescoped weapons. A similar weapon has since been fitted to the CS/VP-16B, a 6×6 all-terrain buggy in service with the PLA, but it is not clear whether it has entered service officially. Strictly speaking, the requirement for a larger calibre weapon to arm its IFVs may not be a driving force at present. The country’s two primary land opponents are unlikely to enter vehicles into service that outstrip the lethality of their 30 mm 2A42 clone within the next decade.
“Nope, already been there.”
The 1990 film Back to the Future Part III ends with the line, “nope, already been there,” when Doc is asked if he will be returning to the future. It seems that medium-calibre lethality is set to return to the state that prevailed during the Cold War. In part, it stands to reason that this is a result of the end of the Soviet Union – most of the articles discussing medium-calibre developments reference the venerable BMP-3 and its frontal armour designed to stop medium-calibre rounds as the primary threat driver. As the Soviet Union ceased to pose a threat, the need for larger calibres became less pressing. There are multiple combat accounts from Iraq, Chechnya, Afghanistan, and Ukraine to show that the extant calibres are certainly very effective. Others indicate that there was even a trend away from the armour piercing ammunition natures, with a greater focus on frangible and training rounds for their reduced likelihood of causing collateral damage in peace support operations.
Gradually the reduction in the size of armed forces has combined with technological changes in the character of warfare to place greater demands upon AFVs. It is now reasonable to expect an IFV to have some form of counter-UAV capability, even if a kinetic kill is likely less effective than electronic attack when smaller drones such as the DJI Mavic are concerned. There is also a well-understood value in the use of ABM against infantry in pretty much any environment. However, is it reasonable to assume that Russia’s horrific acts in Ukraine will drive new developments? It is clear that NATO as a whole is gripped with the need to rebuild land forces and return to a policy of deterrence by denial. However, Russia’s fleet of IFVs continues to be based around the BMP-3, BMP-2, and to a lesser extent the BMD-4M. These vehicles are arguably already overmatched by the ammunition available to 35 mm and even 30 mm cannons in service with NATO. Furthermore, with the exception of the MBT-based T-15, Russia’s IFVs do not tend to place great emphasis on heavy levels of passive protection, that is a characteristic driven mostly by NATO vehicles.
Added to this are the physical limits of medium-weight vehicles. In theory it is possible to armour them against 30 mm and 40 mm weapons, however in practice doing so would mean these vehicles would approach the weight and cost of a tank – especially if protected from all angles – and this may in turn reduce their utility as IFVs. So, whilst the need for a larger calibre weapon has been stated by the US, UK, France, and Russia, it does not necessarily follow that the entire medium-calibre user group will soon be swapping their 30 mm cannons for the CT40, XM913 or 2A91 despite the apparent advantages of those weapons. For many, the weapons they already have are likely sufficient. It is, however, reasonable to assume that if Russia were to adopt the 2A91 on a large scale, that this would in turn drive similar moves across NATO.