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Sabots peeling off DU penetrator in flight

"Sabot" is usual contraction of "Armour Piercing, Discarding Sabot" (APDS) which describes an entirely kinetic anti-tank round. Kinetic anti-tank rounds carry out their task like latter-day cannonballs: they simply strike the target so hard that it breaks.

Armour-piercing rounds have evolved from the simple AP (Armour Piercing) round which was simply a bullet-shaped cannonball.

Adding a toughened cap to the round improved the energy transfer from the round to the target, causing more damage. This was the APC round (Armour Piercing, Capped).

The optimum design of the APC round for penetration varied significantly from the optimum design for ballistic flight, so the APC round gained a ballistic cap to improve its flight (APCBC: Armour Piercing, Capped, Ballistic Cap).

But all AP rounds and variants were limited in their penetration by the muzzle velocity as it left the barrel. The charge ignited, the force of the explosion accelerated the round down the barrel until it left, at which point it flew at muzzle velocity to the target, losing speed through air resistance and friction. Furthermore, increasing the calibre of the gun didn't simply increase the kinetic energy of the round: more and more charge was required to get the increased weight down the barrel at speed. As the size of the warhead increased, so the size of the case to fire it increased until it started to become impossible to carry a decent amount of ammunition in the space available. Also, once the round was fired, the empty case was ejected and left at the bottom of the turret.

The APDS concept derives from a German design during the Second World War. They determined that by tapering the barrel of an anti-tank gun toward the muzzle, Bernouli's Law dictated that the armour piercing round would speed up dramatically, improving its penetration and hence kill capability. Also, the faster the round, the less effect gravity had on it during flight, the less it deviated from the gunner's point of aim and hence the more accurate it was. The downside of this is that these guns wore out their barrels incredibly quickly.

The design was changed so that instead of the barrel being tapered, the round was made too big to go down the barrel, but with a soft outer shell which compressed as the round went down the barrel and increased muzzle velocity using the same principle. The soft (metal or plastic in some designs) outer shell was called a sabot, from the French for basket. When the gun was fired, the bits of sabot would be propelled down the barrel with the tungsten armour-piercing payload, but would have been crushed during the transit and would fall away from the round as it left the barrel, so the round was armour piercing, discarding sabot.

The APDS concept reached its peak with the 120mm gun fitted to Chieftain. During the 1970s, The Royal Armoured Corps lore had it that a T-62 (Russian MBT) was acquired from the Israelis after a Middle East war and taken to be studied. Once everything had been stripped out and examined, the only thing left to do was fire a round at it and see what its survivability was like. The story goes that an APDS round from the Chieftain conducting the experiment struck the side of the T62 side on and the target split in two. Highly satisfactory.

During the 1970s, the Royal Armaments Research and Development Establishment at Chobham in Surrey invented an entirely new form of armour plating which was highly effective at dissipating the energy whose transfer from round to tank was so destructive. This so-called Chobham armour was highly classified for many years, though it was described as comprising of layers of armour plate with other substances which transferred the energy away. The British Government did not even give information about Chobham Armour to the Americans until they had devised a round to defeat it.

This round was a variant of the APDS round, Armour Piercing, Fin-Stabilised, Discarding Sabot (APFSDS). The concept was the same except that the actual warhead was longer, giving better energy transfer upon striking the target, but was less ballistically stable in flight until small fins were added.