AFV ASLAV 8x8 Australian Army with M242 BUSHMASTER cannon
Requirements and technologies
Medium-caliber automatic cannons designed for installation on armored combat vehicles (AFVs) have been constantly evolving over the past decades. This concerns their characteristics and operating principles, as well as their respective operational concepts
In this article, we will briefly highlight the key factors of the growing demand for weapons of this class and the influence of these needs on the choice of the optimal caliber and other characteristics, and then move on to describe the defining technologies of modern models.
Large calibers for growing needs
The first attempts to arm armored combat vehicles with more powerful automatic weapons compared to the then ubiquitous heavy machine guns (M2 12.7 mm in the West and CPV 14.5 mm in the Warsaw Pact countries) began in the late 50s and early 60s in the framework of the general trend "Motorization" of infantry units, which affected all the leading armies of the world.
In the West, initially, this work, as a rule, consisted in the refinement of automatic cannons, originally developed for installation on combat aircraft or anti-aircraft installations. The first turret systems of this type included mainly the Hispano Suiza HS-820 cannon (with a chamber for a 20x139 projectile), which was installed on German SPZ 12-3 vehicles (1,800 vehicles were manufactured for the Bundeswehr in 1958-1962) and the reconnaissance version of the M-114 tracked armored personnel carrier M-113 of the American army. On the other hand, the Russians initially adopted a unique approach, equipping their new BMP-1s (the predecessor of all infantry fighting vehicles) with the 73mm 2A28 Thunder low-pressure cannon, without dividing the Western choice in favor of medium-caliber automatic cannons. However, they appeared on their next generation cars.
However, these first applications of automatic cannons on armored combat vehicles immediately confirmed not only a very important operational need for them, but also revealed the corresponding shortcomings of the weapons then used. Unlike aircraft and anti-aircraft weapons, automatic cannons on armored combat vehicles are used to engage a wide range of targets, from unarmored to fortified and armored, often in the same battle. Accordingly, the presence of a double feed system, which would allow the shooter to quickly switch from one type of ammunition to another, has become mandatory.
The HS-820 was a single-feed cannon, and remained so even after being redesigned and redesignated Oerlikon KAD. For this reason, as well as for industrial policy reasons, in the early 70s, Rheinmetall and GIAT developed and implemented a new generation of 20mm dual feed cannons: the Mk20 Rh202 for the MARDER and the M693 F.1 for the AMX-10P, respectively.
Progressive increase in requirements for armor penetration of BMP cannons as a result of the appearance of enemy vehicles with enhanced protection
KBA cannon from Oerlikon (currently Rheinmetall DeTec) with a chamber for 25x137 ammunition
Comparison of the sizes of the main types of ammunition currently used (or proposed) for automatic cannon BMP. Left to right, 25x137, 30x173, 35x228, 40x365R and telescopic 40x255
CT40 cannon with loader and appropriate ammunition
Both Mk20 and M693 cannons fired a 20 x 139 projectile, but immediately after their appearance, doubts began to arise about the characteristics of these ammunition, which could really meet the rapidly evolving operational needs in terms of effective range, impact of the projectile in the final section of the trajectory and armor-piercing power, especially in the then dominant concept of warfare in Central Europe. In these scenarios, providing fire support to dismounted infantry units was considered primarily from the point of view of engaging enemy light / medium armored combat vehicles. Accordingly, one of the most important characteristics of the fire support required for such weapons was the large penetration capacity at distances of up to 1000 - 1500 m. Currently, the smallest caliber capable of penetrating 25 mm thick armor with an inclination of 30 ° (that is, BMP-1) from 1000 meters, is 25 mm. This led to the fact that several Western armies, primarily led by the United States, missed the generation of 20 mm weapons for their infantry fighting vehicles and switched from 12, 7 mm machine guns directly to weapons with a chamber for the powerful 25 x 137 Swiss projectile. as the first, specially designed automatic cannons intended for installation on infantry fighting vehicles.
Armament firing 25 x 137 ammunition is currently installed on many different tracked and wheeled infantry fighting vehicles, including the American M2 / M2 BRADLEY and LAV25, the Italian DARDO, the Danish M-113A1 with the T25 turret, the Canadian KODIAK, the Spanish VEC TC25, the Turkish ACV, Japanese Type 87, Singapore BIONIX, Kuwaiti DESERT WARRIOR and Australian ASUW.
But "appetite comes with eating" and a couple of leading armies realized that even 25-mm weapons were not powerful enough. This was not so much due to the same big fears that led to the rapid displacement of the 20 mm caliber with the 25 mm caliber, but rather to a wider perception of the role and purpose of the BMP. In addition to fire support for dismounted infantry units, BMPs were seen as an auxiliary combat vehicle for MBT, responsible for engaging targets that do not require large-caliber ammunition, as well as a kind of "mini-MBT" in scenarios with a lower threat level. In this case, a cannon is needed that is capable of firing not only armor-piercing shells, but also high-explosive fragmentation shells with an appropriate explosive charge.
Based on this, the British and Soviet armies made the transition to 30 mm, introducing the RARDEN cannon (30 x 170 ammunition) for the WARRIOR and SCIMITAR vehicles and the 2A42 (30 x 165) cannon for the BMP-2 and BMD-2. Likewise, the Swedish army in the early 80s began a program for its BMP (eventually the CV90) and decided to install a Bofors 40/70 cannon on it, firing powerful 40 x 365R ammunition.
Rheinmetall Mk30-2 / AVM was developed as the main armament of the new German BMP PUMA
Relatively recent incarnations of this concept are the unique two-caliber weapon unit 2K23 from the KBP installed on the Soviet / Russian BMP-3 (automatic 30-mm cannon 2A42 + 100-mm cannon 2A70), and the Rheinmetall Rh 503, originally intended for the "ill-fated" MARDER 2 and a 35 x 228 shot chamber. The latter has the potential to grow further as it can be upgraded to the 50 x 330 "Supershot" telescopic projectile by simply changing the barrel and a few components. Despite the fact that the Rh 503 was never mass-produced, the innovative concept of a quick change of caliber generated interest; it was adopted in particular for the projects BUSHMASTER II (30 x 173 and 40 mm "Supershot") and BUSHMASTER III (35 x 228 and 50 x 330 "Supershot"), although none of the operators of these guns have yet taken advantage of these possibilities …
Currently, there is a kind of general agreement in the sense that 30-mm weapons are the minimum that can be installed on armored infantry fighting vehicles and reconnaissance vehicles of the latest generation. As for the choice of users,then here the newest significant developments were the Type 89 machines with a 35 mm cannon, the Dutch and Danish decision to install a 35 mm cannon on their CV90s, the modernization of the Singapore BIONIX vehicle and the installation of a 30 mm cannon (BIONIX II), the intention of the British army, finally, to certify the CT40 cannon from CTA International (BAE Systems + Nexter), which fires unique telescopic shots 40 x 255, for the modernization of British WARRIOR vehicles (the so-called Warrior BMP extension program - WCSP), as well as for the promising FRES Scout vehicle and, finally, adoption of the South Korean BMP K21 with a local version of the 40/70 cannon.
At least all of the aforementioned European decisions were probably motivated by a return to the emphasis on armor-piercing characteristics, based on the understanding that even 30-mm armor-piercing sub-caliber shells (APFSDS) would not be able to satisfactorily cope at probable ranges with the latest Russian BMP-3s, which have additional booking. In a broad sense, it is important to note that the current deployment of many armies in asymmetric combat scenarios leads to the introduction of increasingly heavy additional armor kits for BMPs. Despite the fact that this additional armor is mainly intended to protect against improvised explosive devices (IEDs) and RPG-type threats, rather than automatic cannon fire, it can be assumed that promising high-class infantry fighting vehicles will need at least 35-40 -mm weapons for successfully fighting modern vehicles of the same class.
And then a puzzle appears. It is quite obvious that the armament of the BMP with a 35-40 mm cannon in the turret already includes certain compromises regarding the combat weight and size of the vehicle (with a direct negative impact on strategic mobility), the permissible ammunition capacity and, most importantly, the number of infantrymen transported. By increasing the caliber further, you can actually create a light tank with a minimum internal space for infantrymen and their standard armament, both individual and squad weapons. If increased armor-piercing capabilities are to be actually perceived as mandatory, perhaps the most practical way to achieve this is to rely solely on ATGMs, whereas a cannon could be optimized mainly, but not exclusively, to destroy unarmored or partially armored targets. Thus, we see a full cycle of return to the BMP-1 philosophy.
As for the progress in ammunition, here the two most significant events were probably the appearance of APFSDS armor-piercing projectiles (armor-piercing subcaliber with a stabilizing shank (feathered)) for 25-mm (and larger) weapons, and the development of high-explosive fragmentation ammunition ABM (Air Bursting Munition - air blast projectile) or HABM technology (high-speed ABM) with an induction electronic fuse; the first here was the Oerlikon AHEAD concept for projectiles from 30 mm and above. These projectiles can effectively hit personnel behind natural shelters.
Apparently, a secondary, but really important issue in connection with the installation of automatic cannons of an armored fighting vehicle is the removal of the fired cartridges, preventing their ricochet inside the fighting compartment, so they become potentially dangerous at the same time. The photo of the DARDO BMP of the Italian army with the Oerlikon KBA 25 mm cannon shows open hatches for ejection of casings
A variant of the ubiquitous anti-aircraft gun Bofors 40/70 is installed on the Swedish CV90 BMP; when installed, it flips 180 degrees
Simplified diagram of a chain-driven cannon concept
Main technical characteristics
Based on the modes of firing powerful ammunition, all automatic cannons for AFVs currently available on the market are rigidly locked, that is, the breech is rigidly locked with the receiver / barrel assembly during firing. This can be achieved by either a rotary bolt with locking protrusions (for example, Oerlikon KBA 25 mm), valves with retractable locking flaps (for example, Rheinmetall Mk20 Rh-202, GIAT MS93 F1), and vertical (e.g. Bofors 40/70) or horizontal (RARDEN) sliding gates. The revolutionary CTA 40 cannon is special in its class, it is characterized by a horizontally rotating (90 degrees) charging chamber, separated from the barrel.
In terms of operating principles, most of the usual practical concepts for such weapons are long recoil, venting, hybrid systems and external power.
The appearance of armor-piercing sub-caliber ammunition 25 x 137 made it possible to significantly improve the armor-piercing characteristics of 25-mm weapons
Prototype BMP WARRIOR with CT40 cannon installed during firing tests
Long pullback
In all weapons, which use recoil forces and rigid locking, the energy required to complete the firing cycle is supplied to the bolt due to the reverse movement of the bolt itself and the barrel, locked together and rolling back under the pressure of powder gases. In a system with a "long rollback", the bolt and the barrel roll back a distance greater than the length of the non-fired projectile. When the pressure in the chamber decreases to acceptable levels, the bolt is unlocked and begins the opening / extraction sequence, while the barrel returns to the forward position, the bolt then also moves forward due to its spring, sends out a new shot and locks it.
This principle offers a certain set of advantages for turret weapons designed to destroy ground targets. The backward movement, being relatively less intense than in the case of the short recoil design, is transformed into lower forces transferred to the mechanisms of the gun and its installation, which increases the accuracy of shooting. In addition, the bolt, locked for a longer period of time, facilitates the removal of powder gases through the muzzle and prevents them from entering the fighting compartment of the vehicle. These advantages come at the price of a relatively low rate of fire, but this is not a significant problem for BMPs.
Typical examples of long recoil weapons are the RARDEN 30mm and Bofors 40/70. It is also interesting to note that two manufacturers that are traditional proponents of off-gas designs, namely the Swiss company Oerlikon (currently Rheinmetall DeTec) and the Russian company KBP, have adopted the concept of a long recoil for weapons specifically designed for installation on BMP (KDE 35 mm for the Japanese Type 89 and 30 mm for the BMP-3, respectively).
Principle of operation due to the removal of gases
Originally developed by John Browning, this system relies on energy generated by the pressure of the powder gases discharged at a point along the barrel. While several variants of this concept are used in hand-held firearms, most of the automatic cannons acting by exhausting gases for infantry fighting vehicles are based either on the principle of a piston, where gases press on a piston, which is directly connected to the bolt and pushes it back, or on the principle exhaust gas, when the gases transfer energy directly to the bolt carrier.
When compared with the direct recoil principle, the advantage of the principle of operation due to the release of gases is that the barrel is fixed (and, therefore, the accuracy is increased), it becomes possible to adjust the firing cycle in accordance with weather conditions and the type of ammunition by appropriately adjusting the gas release valve … On the other hand, the entire gas system must be carefully tailored to prevent toxic powder gases from entering the fighting compartment.
Mixed process
In many automatic cannon designs, gas work is actually associated with other concepts, resulting in what could possibly be called a hybrid (mixed) process (although this is not a universally accepted definition).
The most common solutions combine the work of gas with recoil (thus the energy required to complete the firing cycle acts on the bolt due to the reverse movement of the sleeve caused by gas pressure). The gases emitted from the barrel are used only to unlock the bolt from the receiver, after which the reverse gases push the bolt back. The whole implement then rolls back 20 - 25 mm, this energy is used to operate the feed system.
This principle of "operation of gases + free shutter" allows the use of relatively light and simple mechanisms, which led to the adoption of this principle for Hispano Suiza automatic cannons after World War II (for example, HS-804 20 x 110 and HS-820 20 x 139), as well as several guns from Oerlikon, GIAT and Rheinmetall.
Gas work can also be combined with barrel recoil, as is customary, for example, for the Oerlikon KBA (25 x 137) cannon, originally designed by Eugene Stoner.
The Danish (pictured) and Dutch armies have opted for the ATK BUSHMASTER III cannon, which fires powerful 35 x 228 ammunition. It is also possible to upgrade to the 50 x 330 "Supershot" variant for installation on the new CV9035 infantry fighting vehicles
Twin gun Nexter M693 F1 on the AMX-30 tank. It has a piston mechanism with exhaust gases and a rotary valve with retractable locking shutters
The Rheinmetall Rh 503 cannon pioneered the concept of an automatic cannon, which is capable of firing ammunition of two different calibers by simply replacing the barrel and several components.
Armament with external power supply
The most typical examples of externally powered automatic cannons are probably revolving and Gatling designs, but they are definitely designed to achieve a high rate of fire and are thus not interesting to mount on an AFV. Rather, the externally powered armament mounted on an armored vehicle is mainly intended to make it possible to adapt the rate of fire to the special characteristics of the targets being hit (rate of fire, however, is always lower than that of a similar weapon operating by exhausting gases), while in general the armament this type can be lighter, cheaper and requires less volume for itself. In addition, externally powered weapons are, by definition, free from misfires, since a faulty shot can be retrieved without interrupting the firing cycle.
Critics of the externally powered weapon concept draw attention to the fact that any breakdown and damage to the electric motor and / or power supply can render the gun inoperative. While this is undoubtedly true, at the same time it should be taken into account that a power outage will also disable optoelectronic devices (sights, displays and stabilization systems), in which case armament, working by gas evacuation or operating due to the bestowal, they actually become useless.
"Chain" systems
The Chain Gun (this is a registered trademark, not a generic definition), developed in the early 70s by then Hughes (later McDonnell Douglas Helicopters, later Boeing, now ATK), uses an electric motor to drive a chain that moves along a rectangular contour through 4 stars. One of the chain links is connected to the bolt and moves it back and forth to load, fire and remove and eject the casings. During each full cycle, consisting of four periods, two periods (movement along the long sides of the rectangle) determine the time it takes to move the bolt forward and load the projectile into the chamber and retrieve it. The remaining two periods when the chain moves along the short sides of the rectangle determine how long the bolt remains locked during firing and open to remove the case and ventilate the powder gases.
Since the time it takes for the chain to complete a full cycle in the rectangle determines the rate of fire, the change in engine speed allows the chain gun, in principle, to shoot at a continuous rate varying from single shots to the maximum safe rate of fire, depending on the rate of pressure drop in the barrel after a shot, mechanical endurance and others. factors. Another important advantage is that the design allows for a very short receiver, which makes it easier to install weapons inside the turret.
The most famous and widespread chain guns are the BUSHMASTER series guns, including the M242 (25 x 137), the Mk44 BUSHMASTER II (30 x 173) and the BUSHMASTER III (35 x 228).
Electrical system from Nexter
The Nexter M811 25 x 137 cannon is mainly installed on the new VBCI 8x8 infantry fighting vehicle, and is also in service with the Turkish army (ACV); it is based on a patented external drive concept. An electric motor drives a camshaft inside the receiver, whose rotation locks and opens the bolt as it moves back and forth. This roller is also geared to the feed mechanism so that the loading is precisely synchronized with the movement of the shutter. Shooting modes - single shot, short burst and continuous burst.
Push system
The so-called "Push Through" system, developed by CTA International for its CT 40 armament, uses the most innovative, if not revolutionary, operating principle of all those described in this article. In this case, there is a very strong connection between the principle of operation and the ammunition, in that the "push" concept is strictly dependent on the availability of a telescopic ammunition with a perfect cylindrical shape.
The cylindrical ammunition allows the use of a loading mechanism in which the powder chamber is not part of the barrel, but rather a separate unit that is rotated around the axis by 90 ° by an electric motor for loading. Each new projectile pushes the previous fired cartridge case (hence the "push"), after which the chamber is rotated to align with the barrel for firing. This completely eliminates all of the retrieval / removal sequence required for conventional "bottle" ammunition, resulting in a simpler and more compact loading mechanism and process with fewer moving parts, ideally suited for installation inside a turret. The CT cannon occupies about the same space as a regular 25mm cannon, but at the same time offers much higher performance (for example, the APFSDS armor-piercing round will penetrate steel armor more than 140mm thick). Also, this unique loading mechanism allows the breech to be removed far forward, thereby significantly improving communication between crew members and their “fighting qualities”.
However, it should be noted that this elegant and (apparently) simple principle of operation really requires a careful design and a high culture of production in order to guarantee an overall gas tightness between the powder chamber and the barrel.
Schematic representation of the principle of operation of the CT40 cannon with telescopic ammunition
APFSDS round 35 x 228 (left) and corresponding 50 x 330 "Supershot" ammunition (center and left)
The Rheinmetall RMK30 (pictured during firing tests on a WIESEL transporter) is the world's first recoilless automatic cannon. It has an external drive, a three-chamber revolving design, shoots caseless ammunition 30 x 250, while some of the powder gases are thrown back, compensating for the rollback; this allows for lighter and less durable structures. Although the RMK30 was originally designed for installation in helicopters, it can also be used in combat modules on light armored combat vehicles.
Rheinmetall ABM (air burst munition) air burst munition with a programmable fuse. The projectile has an electronic module that is inductively programmed at the muzzle (compensating for different initial velocities) in order to guarantee accurate delivery of the warhead. ABM ammunition is capable of engaging a wide range of targets on the modern battlefield, including infantry fighting vehicles, ATGM launchers, dismounted troops and helicopters
ATK's BUSHMASTER II cannon is designed for 30 x 173 ammunition, but can be easily converted to fire 40mm Supershot rounds
Modern tendencies
While all the operating principles described above are currently being used simultaneously and in parallel, there is an unmistakable trend in the West towards the adoption of externally powered designs, while the Russians remain loyal to traditional concepts of flue gas operation. As for the choice of caliber, here, in addition to operational considerations, industrial and financial issues also play an important role. In particular, the Bundeswehr is a typical example. The German army initially adopted 20 x 139, in the early 80s deciding to go to 25 x 127, for which they installed a Mauser Mk25 Mod. E cannon in the KuKa tower as an upgrade of their MARDERs. Later, the upgrade was canceled and it was decided to go straight to MARDER 2 with the Rheinmetall Rh503 35 x 288/50 x 330 Supershot cannon, but after the fall of the Berlin Wall and the end of the Cold War, MARDER 2 with its Rh503 was canceled and chose the more acceptable and better balanced Rheinmetall Mk30- 2 30 x 173 for the new PUMA BMP.
Broadly speaking, 20 x 139 is currently the only shell for older vehicles awaiting retirement. The 25 x 137 ammunition is still "valid" as an acceptable compromise between performance and price, but for new generation vehicles or newly ordered vehicles for wheeled models, light weight, compactness and cost are the main arguments here. In fact, 30 x 173 was chosen as the base option when there is no valid reason to have a smaller or larger caliber. It is adopted, for example, for the Austrian ULAN, the Spanish PIZARRO, the Norwegian CV9030 Mk1, the Finnish and Swiss CV9030 Mk2, the prospective US Marine Corps EFV vehicle, the Polish ROSOMAK, the Portuguese and Czech PANDUR II, the Singapore BIONIX II, and many others. The 35 x 228 ammunition is expensive but high performance, while the 40 x 365R also has a couple of fans.
An externally powered Nexter M811 (25 x 137) cannon was adopted for the new VBCI vehicle of the French army.
The real way forward is quite clearly represented not by the CT 40 as such, but of course by the advanced technology that it represents. But whether financial and industrial factors allow these promising benefits to actually be realized and operational status remains to be seen.
Thus, it is very encouraging that ongoing work is underway on the automatic 40-mm weapon system with telescopic ammunition CTWS (cased telescoped weapon system), developed by CTA International, as part of the WARRIOR BMP (WCSP) service life extension programs, the FRES Scout reconnaissance vehicle for the British army and a promising reconnaissance vehicle for the French army. The CTWS weapon system has already fired and has been tested with its original ammunition delivery system, but this year's firing will demonstrate for the first time the capabilities of the CTWS, which will be installed in a full WCSP turret. However, the shooting will more likely be carried out from a stationary position, and not in motion, as previously suggested by representatives of Lockheed Martin UK.
The next step will be negotiations on the serial production of the CT gun (CTWS). BAE Systems Global Combat Systems - Munitions (GCSM), under license from CTAI, recently submitted a proposal to the British Department of Defense for the production of mass-produced ammunition under an existing contract for the supply of MASS ammunition to the UK. The license will also be issued to Nexter Munitions for the production of serial ammunition for the French arms procurement agency.
