Options for Light/Medium AFV Gun Armament
Total Page：16
File Type：pdf, Size：1020Kb
Options for light/medium AFV gun armament © Anthony G Williams Based on presentations given to the DCMS Mounted and Dismounted Close Combat Symposium, July 2015, and the NDIA Armament Systems Forum, April 2016 This is a huge field to cover, so this can be no more than an overview of the issues involved in armament selection. I will not be addressing the armament of self- propelled anti-aircraft systems or of vehicles armed with mortars or large calibre guns of 60+ mm. There are three elements in armament selection: - the gun and its ammunition - the type of mounting - the context: the purpose of the vehicle on which the weapon is mounted; the purpose of the weapon; the expected target set; and the expected mode of employment. This is of course of critical importance but involves various factors which are outside the scope of this presentation. Guns and ammunition Pictures of guns tend to be uninformative so I'll start by showing comparative photos of the ammunition instead, which tell you more about their relative performance, starting with the least powerful group. Rifle Calibre Machine Guns (RCMGs) are primarily for self-defence, while AGLs and HMGs enable a useful measure of fire support to be provided for dismounts. However, while an HMG delivers a high velocity bullet to long range its destructive effects are limited; and while the explosive charge in a 40 mm grenade provides significant destructive power, the velocity is so low that the steep trajectory and long flight time make hitting moving or fleeting targets increasingly challenging as the range lengthens. One attempt to replace both HMGs and AGLs with one lightweight weapon was the Orbital ATK LW25 firing 25 mm calibre grenade ammunition, but this achieved no sales and is no longer advertised. In a similar size and weight class comes the smallest of the cannon, the GA-1 from South Africa, better known as the Mauser MG 151/20, the Luftwaffe's principal aircraft cannon in World War 2. 1 Another, more recent, development from South Africa is the 20mm Inkunzi Strike which fires the same projectiles as the GA-1 but from a much smaller 20 x 42B cartridge case, providing a subsonic muzzle velocity of 310 m/s. Whether you regard it as a large machine gun or a small grenade launcher is a matter of opinion! Cannon ammunition To obtain both a high hit probability through high velocity and rapid fire, combined with significant destructive effects, it is necessary to select a cannon. These come in various calibres between 20 and 57 mm, with muzzle energies ranging from 30 to 1,400 kilojoules. The photo on the next page shows rounds in the 20 to 30 mm group. Over the past few decades there has been a gradual increase in the calibre of the gun and the power of the ammunition used by IFVs. The first generation of cannon- armed troop-carrying armoured vehicles had 20 mm guns but these have mostly been supplanted, although 20 mm guns are still offered in light vehicles, mainly of French or South African origin. The German Marder, now finally being replaced, is one of the last IFVs in a major army still using a 20 mm gun (the Rh 202 in 20 x 139 mm calibre). By the 1980s the baseline gun calibre had become 25 x 137 mm, 2 epitomised by the US Bradley IFV, with the British and Russians stepping up to 30 mm, a calibre which in recent years has become the new baseline. The standard Russian 30 x 165 is slightly less powerful than what has become the NATO standard 30 x 173 round (available in the Mauser MK 30 cannon and the ATK Bushmaster II Chain Gun, now designated the MK44 and further modified for AFV use as the XM813). The Rarden's 30 x 170 ammunition is similar in size and power to the 30 x 173, but is not interchangeable with it and there is a far more restricted range of ammunition natures available. Some countries have gone further: in the 1990s the Japanese Type 89 IFV entered service with a 35 mm Oerlikon gun and the Swedes adopted a 40 mm Bofors for the CV90. More recently, the Danes and the Dutch both chose the 35 mm Bushmaster III for their CV9035, while within the next few years the British Army will begin to receive vehicles armed with the unique 40 mm cased-telescoped weapon system (CTWS): the Ajax reconnaissance vehicle and the upgraded Warrior IFV. The 40 mm Super 40, based on the 30 x 173 case, is currently offered by ATK as an upgrade to the XM813. It is not yet in service, although reportedly an export order has been placed. 3 There are now three choices in 40 mm AFV ammunition on offer: the original Bofors 40 x 365 available in their L/70 gun; the 40 x 255 CTWS which delivers similar performance but is far more compact; and the smaller and less powerful 40 x 180 "Super 40" from the USA. This is based on the 30 x 173 case and can be fired from suitably modified variants of the 30 mm MK44 (Bushmaster II) Chain Gun - the MK44 "Stretch" or the XM813 - although it has so far not achieved any sales. Another option for further development is the 50 x 330 Supershot, which is based on the 35 mm case. Originally developed by Rheinmetall in the 1980s the project was stopped after the end of the Cold War but has recently been advertised by ATK as a Bushmaster III upgrade and has also been mentioned in presentations concerning the US Army's long-term thinking, although if that ever emerges it might take a different form from that shown here, with more conventional proportions. To top them all, Russian firms have recently been marketing an updated version of their old 57 mm AA gun installed in an unmanned turret, the AU-220, for fitting to medium armoured vehicles. To date it is only available with old-fashioned ammunition types, but new loadings are said to be on the way. Obviously, the more powerful the ammunition, the heavier the gun and mounting, and the smaller the quantity of ammunition that can be carried in any given vehicle. 4 The chart below gives another form of comparison, based on the approximate muzzle energies of the rounds. I am not quoting penetration figures because of the problems in finding exactly comparable data, but other things being equal (especially penetrator design) muzzle energy is a good indicator of relative performance in respect of armour penetration potential. You can see that not all 40 mm rounds are equal and that the 35 mm is unusually powerful for the calibre. The Russian 57mm is omitted as it would be way off the scale. One of the factors driving calibre growth has been the increasing protection levels of this class of vehicle, requiring more powerful AP ammunition to achieve reliable penetration. Medium IFVs are now being designed or upgraded to resist 30 mm AP projectiles over the frontal arc. The standard type of AP projectile is now tungsten alloy armour-piercing fin-stabilised discarding sabot (APFSDS). The US Army uses depleted uranium cores which boost penetration by perhaps 10%, while the British and Russian armies still use the older spin-stabilised APDS which is less effective, especially at longer ranges. There is clearly a relationship between the calibre of gun selected and the decision about whether or not to mount anti-tank guided weapons on the turret. With ATGWs 5 to deal with the heavy opposition, the gun could arguably be smaller. But that is a decision with lots of pros and cons which each army needs to determine. Another more recent factor in the growth in calibre may be the introduction of timed airburst ammunition (known as HEAB – high explosive air burst – or PABM – Programmable Air Burst Munition) to engage troops in defilade, in which a very precise time fuze in the shell is set electronically on firing. These fuzes are relatively expensive, so it is logical to maximise "bang for the buck" and use them in bigger projectiles. The chart below shows typical weights of HE ammunition. There is a sales battle going on at the moment between HEAB and Kinetic Energy Time Fuzed (KETF - better known as AHEAD). The 30mm HEAB shell is primarily filled with HE but lined with small pellets, designed to form a spherical pattern of high-velocity fragments (although this arguably works less well with high-velocity projectiles, as the fragments start with a considerable forward velocity). KETF rounds, in both 30 mm and 35 mm, are filled with larger tungsten pellets, expelled mainly in a forwards direction by a small base charge. HEAB is better at catching personnel hiding behind walls or in trenches, KETF delivers a much denser pattern of heavy pellets onto the target and could be effective against even MBTs by damaging their optical sensors. 6 The cannon described so far have all fired high-velocity ammunition relying on kinetic energy to penetrate armour. There is one interesting alternative for use in lighter mountings: the M230LF (Link Fed), a version of the M230 Chain Gun used in the Apache helicopter with a longer barrel and a few other modifications. The smaller and less powerful ammunition relies on the shaped charge element in High-Explosive Dual-Purpose ammunition for armour penetration. HEDP is not as effective against armour as high-velocity APFSDS, so the gun is more suited to a general fire support role.