This is a type of ammunition designed to pierce armour. For the sake of clarity this entry will only describe kinetic armour piercing ammunition - shaped charge rounds are dealt with under their own entry.
Kinetic armour piercing rounds are those which use the kinetic energy imparted to them on firing to defeat the armour of their target - and thus they aim to deliver as much mass as possible, travelling as fast as possible to the smallest target area commensurate with actually killing the target.
Obviously there are compromises to be played out here - accelerating a massive object to a high velocity takes a lot of energy and places a heavy load on the launcher, and the projectile must be carefully manufactured so that it is stable in flight, accurate and doesn't disintegrate on impact.
The earliest purpose built kinetic armour piercing round is probably the bodkin arrow - differing from a normal "broadhead" arrow in being a narrow steel point little wider than the shaft and thus concentrating the full energy of the shaft into a much smaller area. Some sources indicate a similar ballista tip (confusingly almost nothing to do with an actual ballista) developed later to further improve anti-armour performance but this remains disputed for the time being. The crossbow delivered a similar looking round … and then firearms came along, and everything was turned upside down for a while.
The next time armour became a problem was in the matter of naval gunnery - as the ironclad warship started to become common it became increasingly clear that old fashioned cannon balls weren't going to work on them. Developement started on modern armour piercing rounds.
Much like the bodkin they were made of steel and pointed at the end, all the better to punch through iron plate. They were generally fired with a larger charge than regular balls, which tended to demand a stronger gun - and which lead to some truly peculiar looking pieces as gunsmiths cast cannon with massively thickened breech ends. The new rounds also benefitted from being fired from rifled guns with long barrels, giving the round as long as possible to accelerate under the influence of the charge.
And there things stayed for a while … until the tank appeared on the battlefield. Suddenly the gunsmiths had the challenge of hitting as hard as possible, whilst also being concerned that the gun should be as light as possible.
To begin with, the guns just got heavier - the same design of rounds, bigger to carry more mass and pushed harder down longer barrels - but it quickly became clear that this couldn't last. Also, the high velocities that were being obtained by modern guns meant that steel rounds had a tendency to shatter on impact and the growing use of sloped armour made rounds more likely to slide off. New ideas were needed … and sooner or later, they developed:
- Tungsten (and later depleted uranium) as a material for making rounds. Denser and harder than the best steels it could carry more energy at higher velocities … if you could get hold of it in the first place.
- APDS - a sub calibre dart, accelerated down a much larger gun by a carrier of lighter material, allowing a much larger charge to be used proportionate to the mass of the round.
- APCR - an ancestor of APDS in which the light metal carrier was not detachable.
- Taper Bore Guns - a small family of weapons in which the barrel narrowed from breech to muzzle, meaning that an APCR round fired from it was forced to squeeze down the barrel by deforming the light alloy carrier. Allowed the maximum possible energy to be extracted from the charge.
- Flat nosed rounds (the better to grip sloped armour), topped with an aerodynamic cap of light alloy (known as APCBC for those keeping score).
Some armour piercing rounds also carried an explosive follow up charge, designed to detonate inside the target (these were generally called APEx, APHE or something similar) - they tended to sacrifice penetrating capacity for effect on target and so were weeded out as penetration became paramount and shaped charge rounds took over for dealing with lighter armoured targets.
Modern ordnance tends to either use shaped charge rounds or some variant of APDS, most other designs having gone by the board in the years since the end of WW2. EFP Rounds are covered with the shaped charges, despite having a substantial kinetic component1.
This was the same period in which personal armour became viable again in the shape of the ballistic vest and AP ammunition for small arms suddenly became interesting as well. The armour piercing bullet however has, as yet, not evolved much past the fast-and-pointy stage and remains in fairly limited demand for most applications.