Armour-piercing discarding sabot
Armour-piercing discarding sabot (APDS) is a type of kinetic energy projectile
Kinetic energy penetrator
A kinetic energy penetrator is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target....

 fired from a gun to attack armoured
Vehicle armour
Military vehicles are commonly armoured to withstand the impact of shrapnel, bullets, missiles, or shells, protecting the personnel inside from enemy fire. Such vehicles include tanks, aircraft, and ships....

 targets. APDS rounds are sabot
A sabot is a device used in a firearm or cannon to fire a projectile, such as a bullet, that is smaller than the bore diameter, or which must be held in a precise position. The term is also applied to a battery stub case, a device used similarly to make a small electrical battery usable in a...

 rounds and were commonly used in large calibre tank gun
Tank gun
A tank gun is the main armament of a tank. Modern tank guns are large-caliber high-velocity guns, capable of firing kinetic energy penetrators, high explosive anti-tank rounds, and in some cases guided missiles. Anti-aircraft guns can also be mounted to tanks.-Overview:Tank guns are a specific...

s, but have now been superseded by armour-piercing fin-stabilized discarding sabot (APFSDS) projectiles in such guns. However, APDS rounds are still commonly used in small or medium calibre weapon systems. For a given calibre, this type of ammunition can effectively double the armour penetration of a gun, compared to those firing armour-piercing (AP), AP - capped (APC), or APC + ballistic cap (APCBC) projectiles.

History and development

APDS was developed by engineers working for the French Edgar Brandt company, 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 APDS development efforts there, culminating in significant improvements to the concept and its realization.The APDS projectile type was further developed in the United Kingdom
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 between 1941-1944 by Permutter and Coppock, two designers with the Armaments Research Department. In mid-1944 the APDS projectile was first introduced into service for the UK’s QF 6 pdr
Ordnance QF 6 pounder
The Ordnance Quick-Firing 6-pounder 7 cwt, or just 6 pounder, was a British 57 mm gun, their primary anti-tank gun during the middle of World War II, as well as the main armament for a number of armoured fighting vehicles...

 anti-tank gun and later in September 1944 for the 17 pdr 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 vehicle
Armoured fighting vehicle
An armoured fighting vehicle is a combat vehicle, protected by strong armour and armed with weapons. AFVs can be wheeled or tracked....

s. 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 ~850 m/s when uncapped.
To allow increased impact velocity, a stronger penetrator material was required. The chosen new penetrator material was tungsten carbide
Tungsten carbide
Tungsten carbide is an inorganic chemical compound containing equal parts of tungsten and carbon atoms. Colloquially, tungsten carbide is often simply called carbide. In its most basic form, it is a fine gray powder, but it can be pressed and formed into shapes for use in industrial machinery,...

 (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 Armor-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
Sectional density
Sectional density is the ratio of an object's mass to its cross-sectional area. It conveys how well an object's mass is distributed to overcome resistance. For illustration, a needle can penetrate a target medium with less force than a coin of the same mass...

 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 sabot is a device used in a firearm or cannon to fire a projectile, such as a bullet, that is smaller than the bore diameter, or which must be held in a precise position. The term is also applied to a battery stub case, a device used similarly to make a small electrical battery usable in a...

 (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.

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
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

 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, friction between the pot and sub-projectile allows spin to be transferred, so 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
Rifling is the process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...

 in the rifled bore. This forces the petals tightly against the core, increasing the friction between them and allowing the spin to be transferred.

Sub-projectile construction

The sub-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 used high strength shock resistant tungsten alloys. The main constituent is tungsten, alloyed or sintered with/to cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....

, copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...

, iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 or nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...

. Very few APDS use depleted uranium
Depleted uranium
Depleted uranium is uranium with a lower content of the fissile isotope U-235 than natural uranium . Uses of DU take advantage of its very high density of 19.1 g/cm3...

 (DU) titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....

 alloy for the penetrator material, though the retired 20 mm MK149-2 Phalanx
Phalanx CIWS
The Phalanx CIWS is an anti-ship missile defense system. It is a close-in weapon system and was designed and manufactured by the General Dynamics Corporation, Pomona Division...

 round did use DU.

Sabot discard

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 terminal 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
Royal Ordnance L11A5
The Royal Ordnance L11A5 is a 120 mm L/55 rifled tank gun design. It was the first of NATO's 120mm tank guns which became the standard calibre for Western tanks in the later period of the Cold War...

, as used on the Chieftain tank
Chieftain tank
The FV 4201 Chieftain was the main battle tank of the United Kingdom during the 1960s and 1970s. It was one of the most advanced tanks of its era, and at the time of its introduction in 1966 had the most powerful main gun and heaviest armour of any tank in the world...

, 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
Rolled homogeneous armour
Rolled homogeneous armour is a type of steel which is used to armour vehicles.-Composition:Armoured steel must be hard yet impervious to shock in order to resist high velocity metal projectiles. Steel with these characteristics is produced by processing cast steel billets of appropriate size and...

 at 1000 m.


Many newer medium calibre APDS cores use a frangible high density alloy, the resulting projectiles are called Frangible
A material is said to be frangible if through deformation it tends to break up into fragments, rather than deforming plastically and retaining its cohesion as a single object...

 Armour Piercing Discarding Sabot (FAPDS
Kinetic energy penetrator
A kinetic energy penetrator is a type of ammunition which, like a bullet, does not contain explosives and uses kinetic energy to penetrate the target....

) for APDS types, or FRAP (Frangible Armour Piercing) for full calibre projectiles. During penetration, a frangible projectiles’ 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
High explosive incendiary
In warfare, High-explosive incendiary is a type of ammunition specially designed to pierce armor, fragment, and ignite readily combustible materials....

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.
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