Before firing, the sub-projectile and sabot are locked together. The base unit, if a tracer element is attached to the sub-projectile, has a hole located at the centre. The rear half has a rubber obturator and driving band (again nylon) held in place by the screw-in base unit. The front part of the pot has 3-4 petals (sabots) which are covered with a centering band (often a nylon derivative). The sabot of a large calibre APDS consists of a light high strength alloy full diameter pot and base unit, which is screwed together. Unfortunately the low sectional density of the APCR resulted in poor carrying power (high aerodynamic drag), losing velocity and penetration rapidly over distance. 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. The APCR projectile was about half the weight of a standard AP shot, but of the same diameter. The name given to this projectile type was the Armour-Piercing Composite Rigid (APCR). To overcome this, a lightweight full diameter carrier shell was developed to sheathe the inner high density core. 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. 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. To allow increased impact velocity, a stronger penetrator material was required.
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