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 CIWS 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, centring 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-projectile's higher sectional density, gives the resulting sub-projectile vastly reduced aerodynamic drag in comparison to full-calibre tungsten-core projectiles, such as armour-piercing, composite rigid (APCR). Both the higher initial velocity and the reduced drag result in higher 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