a sabot that was discarded immediately after leaving the barrel, so the smaller, heavier, sub-projectile could carry on at the much higher velocity, suffering less drag due to its smaller diameter. In mid-1944, the resulting APDS (the 'D' standing for "discarding") projectile type was introduced into service for the UK's 57mm QF 6 pounder anti-tank gun and later in September 1944 for the 76mm QF 17 pounder anti-tank gun. 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 three-four petals (sabots) which are covered with a centring 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-calibre projectile consists of a high density core with a penetrating cap, enclosed within a high strength sheath (steel) with a lightweight alloy (aluminium-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.