Armor Piercing Fin Stabilized Discarding Sabot

The Armor Piercing Fin Stablized Discarding Sabot (APFSDS) round or kinetic energy penetrator (also known as a KE weapon) is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target.

This is a special kind of round that uses extremly high velocity to pierce armor or hard targets, such as a Spartan

The principle of the kinetic energy penetrator is that it uses its kinetic energy, which is a function of mass and velocity, to force its way through armour. The modern KE weapon maximises KE and minimises the area over which it is delivered by:

concentrating the force in a small impact area while still retaining a relatively large mass
 * Being fired with a very high muzzle velocity
 * Maximizing the mass of whatever (albeit small) volume is occupied by the projectile—that is, using the densest metals practical, which is one of the reasons depleted uranium is often used.
 * This has led to the current designs which resemble a long metal arrow.

History
The first cannons fired kinetic energy ammunition. First these were round balls of worked stone, then round balls of metal. From the beginning, combining high muzzle energy with projectile density and hardness have been the foremost factors in the design of such weapons. Similarly the foremost purpose of such weapons has generally been to defeat armour or other defensive structures, whether stone castle walls, ship timbers, or modern tank armour. Chemical energy ammunition in its various forms has consistently been the choice for those weapons which due to various factors of their design could not generate the high muzzle energy needed by a kinetic energy weapon.

The development of the modern KE penetrator combines two aspects of artillery design; high muzzle velocity and concentrated force. High muzzle velocity is achieved by using a projectile with a low mass and large base area in the gun barrel. Firing a small size projectile wrapped in a lightweight outer shell, called a sabot, raises the muzzle velocity. Once the shell clears the barrel, the sabot is no longer needed and falls off in pieces. This leaves the projectile traveling at high velocity with a smaller cross-sectional area and reduced aerodynamic drag during the flight to the target (see external ballistics and terminal ballistics). Germany developed modern sabots under the name "Treibspiegel" ("Propulsion mirror") to give extra altitude to their anti-aircraft guns during the Second World War. Before this, primitive wooden sabots had been used for centuries in the form of a wooden plug attached to or breech loaded before cannon balls in the barrel, placed between the propellant charge and the projectile. The name "sabot" is the French word for clog (a wooden shoe traditionally worn in some European countries).

Concentration of force into a smaller area was attained by replacing the single metal (usually steel) shot with a composite shot using two metals, a heavy core (based on tungsten) inside a lighter metal outer shell. These designs were known as Armour Piercing Composite Rigid (APCR). On impact, the core had a much more concentrated effect than plain metal shot of the same weight and size. However, the air resistance and other effects were the same as for the shell of identical size.

Between 1941 and 1943, the British combined the two techniques in the Armour-piercing discarding sabot (APDS) round. The sabot replaced the outer metal shell of the APCR. While in the gun the shot had a large base area to get maximum acceleration from the propelling charge but once outside, the sabot fell away to reveal a heavy shot with a small cross-sectional area.

Modern design
The APDS was initially the main design of KE penetrator. The logical progression was to make the shot longer and thinner to concentrate the kinetic energy in a smaller area. However a long, thin rod is aerodynamically unstable; it tends to tumble in flight and is less accurate. Traditionally, shells were given stability in flight from the rifling of the gun barrel, which imparts a spin to the round. Up to a certain limit this is effective, but once the projectile's length is more than 6 or 7 times its diameter, rifling becomes less effective. Adding fins like the fletching of an arrow to the base gives the round stability, hence Armour-Piercing Fin-Stabilized Discarding Sabot (APFSDS). The spin from rifling decreases the effective penetration of these rounds (rifling diverts some of the linear kinetic energy to rotational kinetic energy, thus decreasing the round's velocity and impact energy) and so they are generally fired from smoothbore guns; a practice that has been taken up by Israel --a major supplier of "arrow" rounds-- China, France, Germany, Soviet Union/Russia and the United States in their tanks. APFSDS can still be fired from rifled guns but the sabot is of a modified design incorporating bearings to isolate the spin of the sabot in the barrel from the round itself, so far as practicable. Rifled guns have been kept in use by some nations (the UK and India, for example) because they are able to fire other ammunition such as HESH rounds with greater accuracy. However, the rifling wears down under regular APFSDS use and requires more maintenance. For these reasons the British Challenger 2 is being refitted with a Rheinmetall 120 mm Gun smoothbore gun.

KE penetrators for modern tanks are commonly 2-3 cm in diameter, and 50-60 cm long; as more modern penetrators are developed, their length tends to increase and the diameter to decrease. However the development of heavy forms of reactive armour designed to shear long rod penetrators has prompted the reversal of this trend in the newest US rounds. To maximise the amount of kinetic energy released on the target, the penetrator must be made of a dense material, such as tungsten or depleted uranium (DU) alloy (Staballoy). The hardness of the penetrator is of less importance, but is still a factor as abrasion is a major component of the penetrator defeat mechanism. As DU is itself not particularly hard, it is alloyed with nickel, zinc, or both. DU is pyrophoric; the heated fragments of the penetrator ignite after impact on contact with air, setting fire to fuel and/or ammunition in the target vehicle, thereby compensating for the lack of an explosive warhead in the penetrator. Additionally, DU penetrators exhibit significant adiabatic shear band formation. During impact, fractures along these bands cause the tip of the penetrator to continuously shed material. This erosion maintains the tip's conical shape and increases the amount of pyrophoric fragments released behind the target armour. Other materials such as unjacketed tungsten tend to deform into a less effective rounded profile, an effect called "mushrooming".

Typical velocities of APFSDS rounds vary between manufacturers and muzzle length/types. As a typical example, the American General Dynamics KEW-A1 has a muzzle velocity of 1,740 m/s (5700 ft/sec).[1] This compares to 914 m/s (3000 ft/sec) for a typical rifle (small arms) round. APFSDS rounds generally operate in the range of 1,400 to 1,850 m/s. The sabots also travel at such a high velocity that upon separation, they may continue for many hundreds of metres at speeds that can be lethal to troops and damage light vehicles.

The counterpart of APFSDS in rifle ammunition is the saboted flechette. A rifle firing flechettes, the Special Purpose Individual Weapon, was under development for the US Army, but the project was abandoned.

APFSDS in Halo
APFSDS rounds are most commonly known for appearing in Halo in the Sniper Rifle System 99C-S2 AM and the Sniper Rifle System 99D-S2 AM. These weapons fire a 14.5x114mm cardige at high velocity and leave a noticable smoke trail. When used in skillful hands the APFSDS round can take out even a shielded enemy with one well placed shot.