Light AFV guns and the WCSP and FRES Scout projects

The cannon mounted in the turrets of LAFVs (Light Armoured Fighting Vehicles) have two principal uses: one is to engage their equivalents, for which they need guns powerful enough to penetrate their armour when using AP ammunition, while the other is to engage softer targets; unarmoured vehicles, buildings and other cover, and troops. 

A gun-armour race has been slowly developing among LAFVs. The previously common 20mm calibre, particularly the 20x139 used in the Rheinmetall Rh 202 among others, has mainly been superseded for this purpose by the 25x137 NATO round, principally in the ATK M242 Bushmaster Chain Gun, but the 30mm calibre is now standard for almost all new developments. The British 30mm Rarden L21A2 gun (30x170) has of course been in service for decades, as have the Russian BMP 30mm guns (30x165), and these have more recently been joined in service by the Mauser MK 30 and the ATK Bushmaster II (since replaced in production by the marinised MK44 version developed for the USMC's EFV), both in 30x173 calibre. One oddity is the 1950s Soviet 30x210B cartridge, developed for the NN-30 naval gun, which was adopted by Yugoslavia in the 1980s for the Zastava M86 (single ammo feed) and M89 (double feed) AFV guns; this has been revived for new Serbian LAFV developments.

M2A2 Bradley with 25mm M242 Bushmaster
Moving up in power, the 35x228 Oerlikon round as used in the Oerlikon KDE has been in service for some years in the Japanese Type 89 MICV. The Netherlands and Denmark have also chosen this round for their new CV9035 MICV, only this time in ATK's Bushmaster III. The 40x365R Bofors L70 has been in Swedish service for some years in the CV9040, and the round is also used by the new Korean Infantry Fighting Vehicle. In the late 1970s, Germany considered the idea of a Marder MICV armed with a version of the Bofors 57mm in 57x438R calibre, but this went nowhere.   
 A few years later Otobreda of Italy and IMI of Israel co-developed the self-loading 60mm HVMS 60 (High-Velocity Medium-Support) around new 60x410R ammunition. After a while, the two firms parted company and continued separate developments of the gun and ammunition. The only sale achieved so far is by IMI to Chile, to rearm some old tanks (apparently with a manually-loaded version of the gun). 

The largest conventional cannon currently being promoted is the new Russian AU-220 turret containing a version of the old 57mm S-60 AA gun in 57x347SR calibre. This is initially intended for rearming the PT-76 light amphibious tank but is said to be suitable for other LAFVs. 

For attacking other LAFVs the ammunition of choice has developed from the APHC (armour piercing hard core) through APDS, and is now APFSDS (Armour-Piercing Fin-Stabilised Discarding-Sabot), effectively a miniature version of the principal Main Battle Tank AP ammunition. The penetrators are almost universally of tungsten alloy, although the USA fields the 25mm M919 DU round; the enhanced penetration which this provides helps to compensate for the relatively low power of the cartridge. 

In current service, the AP ammunition is supplemented by point-detonating fuzed HE for use against soft targets. However, the main focus for development in LAFV ammunition at the moment is airburst HE, using a time fuze.

Several drivers are pushing up the gun calibre of new LAFVs. One of them is that the armour protection of such vehicles is improving, as can clearly be seen as a result of operations in Iraq. The weight of existing LAFVs has been steadily increasing, mainly to add protection: over their lifetime, the M2 Bradley has increased from 23 to 30 tonnes, the Warrior from 25 to 32, the CV90 from 21 (prototype) to 35, the German Marder from 27.5 to 37.5, while the new German Puma weighs in at a massive 43 tonnes. This will require more powerful AP ammunition to achieve reliable penetration in the future. 

The need to blow holes in buildings being used as cover also favours larger HE shells. But perhaps even more important is the current interest in airburst HE/fragmentation for attacking enemy forces hiding behind walls and other cover. This is known as HEAB (High Explosive Air Burst) or ABM (Air-Burst Munition). First in the field was a modified version of Oerlikon's AHEAD time-fuzed shrapnel ammo, available in 35mm and 30mm calibre and now redesignated KETF (kinetic energy time fuzed). 
The 35mm KETF is available in a special anti-personnel loading containing 341 cylindrical tungsten sub-projectiles each weighing 1.5g (compared with 152 at 3.3g for the AA version). However, this only throws the fragments forwards, which may miss soldiers behind cover. Accordingly, there is more interest in HE/fragmentation shells which can be designed to project fragments downwards and even backwards, as well as forwards. If one of these shells, with a spherical fragmentation pattern, is detonated above a target, then only a small minority of the fragments will strike the targets. In these circumstances, shells big enough to produce a large quantity of fragments are clearly advantageous (especially as the time-fuze systems are very expensive, so maximising the "bang for the buck" is important). 

As a result of these issues, the minimum calibre being considered for most new LAFV developments is 30mm. Even that may be considered marginal in the long run, hence the current interest in several armies in 35mm and 40mm armament. This provides a measure of future-proofing; a wise precaution given that once a decision about a new gun calibre is made, it tends to be in service for several decades. It is significant that the Dutch study which led to the decision to select the 35mm calibre concluded that 30mm APFSDS would be inadequate to deal satisfactorily with the latest up-armoured versions of the Russian BMP-3. However, there are practical limitations on gun and ammunition size, especially in vehicles intended to carry troops as well as a gun. Perhaps of most significance is that the fact that many programmes will be to fit – or refit – new armament to existing vehicles, in which case the space available for the turret (and especially the diameter of the turret ring) can impose a significant limitation on the size of the gun and its ammunition feed.
There is therefore a trend to try to squeeze more performance out of existing guns by increasing their calibre. An example of that is the development of the 40mm ‘Super 40’, which is basically a necked-out version of the 30x173 case retaining the same rim diameter and overall length as the 30mm cartridge. It is therefore in principle a straightforward task to modify the externally-powered 30mm MK44 gun to take the Super 40 ammunition; it just needs a new barrel and some modifications to the feed unit. A few years ago, General Dynamics Ordnance and Tactical Systems (GD-OTS) were looking at different case lengths for the HE (164mm) and APFSDS (218mm) versions of the Super 40, but more recently have settled on a compromise case length of 180mm for both. At the same time the calibre has been reduced to 39mm to provide more case taper, but it is still referred to as a 40mm round. An earlier, similar, exercise was to neck out the 35x228 Oerlikon case to create the 50x330 ‘Supershot’. However, the Super 40 and the 50mm Supershot have not so far proceeded beyond the development stage: work on the latter has been stopped entirely, and while development of the Super 40 the former had been proceeding at a low level, GD-OTS seem to have recently accelerated work.
There is one Western programme which offers an alternative approach to the LAFV armament problem of providing high performance within compact dimensions: the Franco-British 40mm CTAS (Cased Telescoped Armament System) developed by CTA International, a joint project between Nexter (formerly GIAT) and British Aerospace. This uses very short, telescoped ammunition just 255mm long overall (the projectiles are entirely enclosed within the case) with a very high performance, approximately equal to that of the 40mm Bofors and the 50mm Supershot (all three cases having similar propellant capacities). The rim diameter of the CTAS round is the same as that of the Bofors 40mm case, but the Bofors round is twice as long. The gun installation is also designed to minimise turret intrusion: the ammunition is fed in sideways (pushing out the fired case as it does so) then the chamber rotated in line with the barrel to fire. The feed is on the axis of the trunnions, so does not move as the gun is elevated. There are clearly some significant advantages here, particularly in minimising the risk of failures to feed and in releasing a lot space in the turret (which looks quite empty compared with a conventional gun installation), although its competitors point out that the trunnion loading means that the gun is out of balance requiring more power for the elevation system, and argue that barrel wear is higher and the ammunition more expensive. However, the higher cost of larger calibre ammunition, plus the smaller quantity which can be carried, is counteracted by the fact that fewer of them would need to be fired to achieve the same effect.
The 40 CTAS HE projectiles weigh 1kg, about 1.5x more than the Super 40 and 2.5x more than the 30mm HEAB: muzzle velocities are similar at around 1,000 m/s. The APFSDS projectiles for the two 40mm rounds are launched at about 1,500 m/s, but again the 40 CTAS is heavier, thereby providing significantly more muzzle energy than the Super 40 (around 500,000 joules compared with c.340,000). However, the Super 40, at 44mm diameter, is significantly slimmer than the 65mm diameter 40 CTAS round, so a lot more ammo can be carried in the same volume.
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