Armour-piercing discarding sabot

Armour-piercing discarding sabot (APDS) is a type of spin-stabilized kinetic energy projectile for anti-armour warfare. It consists of a sub-caliber round outfitted with a sabot to increase velocity compared to a full caliber round by firing a smaller lighter projectile from a relatively larger propellant-charge. The higher velocity gives the round increased penetration against armour. To not break at high speed impacts, APDS rounds traditionally features a hardened core made by tungsten or other hard material.
For a given calibre, APDS ammunition can effectively double the armour penetration of a gun when compared to full-caliber rounds such as AP, APC, and APCBC projectiles.
APDS-rounds were commonly used in large calibre tank guns, up until the early 1980s, but have since been superseded by APFSDS (armour-piercing fin-stabilized discarding sabot) projectiles, which features fin-stabilization and can be fired from smoothbore guns. However, APDS rounds are still commonly used in small or medium calibre weapon systems.
APDS-style ammunition also exists for small arms calibers, known as saboted light armour penetrator (SLAP) ammunition.
History and development.
The genesis of APDS was development by engineers working for the French Edgar Brandt company of saboted ammunition, in which a sub-calibre core was surrounded by a lightweight "sabot" (from the French term 'shoe') that was retained by the round for the duration of its time in flight until impact, and was fielded in two calibers (75 mm/57 mm for the Mle1897/33 75 mm anti-tank cannon, 37 mm/25 mm for several 37 mm gun types) just before the French-German armistice of 1940. The Edgar Brandt engineers, having been evacuated to the United Kingdom, joined ongoing anti-tank ammunition development efforts there, culminating in significant improvements to the concept and its realisation. Whilst the adoption of the sabot improved the performance of the ammunition, the retention of the sabot until impact contributed additional drag that caused the performance of the shot to fall off dramatically with increasing range. What was needed was a sabot that could be discarded after leaving the barrel so that the smaller, heavier, sub-projectile could carry on at the much higher velocity imparted to the whole round while suffering less drag due to the smaller diameter and hence lower frontal area. The resulting APDS—the 'D' standing for "discarding"—projectile type was developed in the United Kingdom between 1941–1944 by Permutter and Coppock, two designers with the Armaments Research Department (ARD). In mid-1944 the APDS projectile was introduced into service for the UK's QF 6 pounder anti-tank gun and later in September 1944 for the QF 17 pounder anti-tank gun. 
The reason for the development of the APDS was the search for anti-tank projectiles with increased penetrating performance. It was known that high impact (terminal) velocity, or a larger diameter projectile would be required to improve penetration. A larger projectile would require a completely new weapon system and may have been too heavy to retrofit onto existing armoured fighting vehicles. Increasing the velocity of the current projectiles was also a problem due to the impact velocity limitations of steel armour-piercing (AP) projectiles, which would shatter at velocities above about 850 m/s when uncapped.
To allow increased impact velocity, a stronger penetrator material was required. The chosen new penetrator material was tungsten carbide (WC), due to its greater hardness and its ability to withstand the greater shock and pressure generated during a higher velocity impact. As the density of WC (≈15.7 g/cm³) is twice that of steel (≈7.86 g/cm³), such a shot was too heavy at full bore to be accelerated to a sufficient muzzle velocity. To overcome this, a lightweight full diameter carrier shell was developed to sheathe the inner high density core. The name given to this projectile type was the Armour-Piercing Composite Rigid (APCR). The APCR projectile was about half the weight of a standard AP shot, but of the same diameter. Due to the large surface area for the gases to impinge upon the lightweight APCR projectile, it experienced a higher average acceleration in the gun barrel, in turn imparting a higher muzzle velocity. Unfortunately the low sectional density of the APCR resulted in poor carrying power (high aerodynamic drag), losing velocity and penetration rapidly over distance.
To overcome these limitations the British devised a way for the outer sheath to be discarded after leaving the bore. The name given to the discarded outer sheath was the sabot (a French word for a wooden shoe). For APDS projectiles the sabot is also known as a pot, as the sabot resembles a flower pot in shape. The APDS has the advantages of the lightweight projectile with regards to bore acceleration and high muzzle velocity, but does not suffer from the high drag of the APCR in flight.
Construction.
Sabot construction.
The sabot of a large calibre APDS consists of a light high strength alloy full diameter pot and base unit, which is screwed together. The front part of the pot has 3-4 petals (sabots) which are covered with a centering band (often a nylon derivative). The rear half has a rubber obturator and driving band (again nylon) held in place by the screw-in base unit. The base unit, if a tracer element is attached to the sub-projectile, has a hole located at the centre. Before firing, the sub-projectile and sabot are locked together. Due to the high setback forces (g-forces), friction between the pot and sub-projectile allows spin to be transferred, thus stabilising the sub-projectile. Small/medium calibre APDS use a lightweight high strength alloy base pot and three or more plastic petals. To transfer the spin to the core in small/medium calibre weapons, the core tends to have a notch at its base. Under bore acceleration, which can be higher than 100,000 g, the uneven base is forced into the softer pot material, locking the sub-projectile to the pot and imparting spin. Not all small/medium calibre APDS rely on this technique, another method for spin coupling is by using the forward plastic petals. The petals are of a slightly larger diameter than the lands in the rifled bore. This forces the petals tightly against the core, increasing the friction between them and allowing the spin to be transferred.
Projectile construction.
The sub-caliber projectile consists of a high density core with a penetrating cap, enclosed within a high strength sheath (steel) with a lightweight alloy (aluminum-magnesium alloy) ballistic cap. For modern small/medium calibre APDS projectiles, the core is not sheathed and the ballistic and penetrating caps are combined. A tracer element may be added to the APDS sub-projectile, for large calibre weapons this is part of the outer sheath, for small/medium calibre weapons it is contained within a hollow cavity in, or attached to, the base of the core. Most modern APDS projectiles use high strength shock resistant tungsten alloys. The main constituent is tungsten, alloyed or sintered with/to cobalt, copper, iron or nickel. Very few APDS use depleted uranium (DU) titanium alloy for the penetrator material, though the retired 20 mm MK149-2 Phalanx round did use DU.
Function.
Discarding of sabot.
When a large calibre APDS is fired and while still within the bore, the setback forces shear the forward petals, partly unlocking the sub-projectile from the sabot, but still holding it rigidly within the pot. Gas pressure is used to delay the unlocking of the pins holding the rear part of the sub-projectile by gyroscopic forces. Once outside the barrel, the pins, centering band and forward petals are released or discarded by projectile spin, the aerodynamic drag removes the pot/base unit. As an APDS sub-projectile does not require driving bands and the core is supported at the base and ogive region, a far more aerodynamic projectile shape can be chosen. This, in combination with the sub-projectiles' higher sectional density, gives the resulting sub-projectile vastly reduced aerodynamic drag in comparison to the APCR. Both the higher initial velocity and the reduced drag result in high velocity at impact. This also lowers flight time and improves accuracy. Accuracy can suffer if there are unwanted sabot/sub-projectile interactions during discard.
Impact example.
The sequence upon impact of the APDS projectile, for example the 120 mm L11, as used on the Chieftain tank, fired L15 APDS (muzzle velocity 1370 m/s), goes as follows: the lightweight ballistic cap is crushed, the penetrating cap then strikes the armour, distributing the shock across the whole surface of the core's nose, reducing the initial shock experienced by the core. The steel sheath surrounding the core peels away, and the core goes on to penetrate the armour. The penetration of the L15 APDS is approximately 355 mm of rolled homogeneous armour at 1000 m.
FAPDS.
Many newer medium calibre APDS cores use a frangible high density alloy, the resulting projectiles are called Frangible Armour Piercing Discarding Sabot (FAPDS) for APDS types, or FRAP (Frangible Armour Piercing) for full-calibre projectiles. During penetration, a frangible projectile's core fragments into many high-velocity pieces. The effect of a frangible projectile on a lightly armoured target is much the same as a high explosive incendiary round, but with a cloud of dense, high-velocity fragments penetrating deeper into the target's interior. Upon striking heavy armour the effect of FAPDS is more akin to a standard APDS, albeit with higher fragmentation of the core, and hence lethality if the armour is perforated.
FAPDS is also known as a Penetrator with Enhanced Lateral Effect (PELE).
References.
 