Armour - piercing discarding sabot Armour - piercing discarding sabot ( APDS ) is a type of kinetic energy projectile fired from a gun to attack armoured targets . APDS rounds are sabot rounds and were commonly used in large calibre tank gun s , but have now been superseded by armour - piercing fin - stabilized discarding sabot ( APFSDS ) projectiles in such guns .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 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 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 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 ( 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 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 . 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 force s , 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 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 , 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 . 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 , 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 ) .