A kinetic energy penetrator, long-rod penetrator, or APFSDS (Armour Piercing Fin Stabilized Discarding Sabot) is a type of ammunition which, like a bullet, does not contain explosives, but uses kinetic energy to penetrate the target. The term is used for more powerful projectiles than ordinary bullets: they are capable of penetrating armour due to:

• being fired with a very high muzzle velocity
• concentrating the force on a small area without having too small a mass, and therefore having the form of a long sleek rod

To produce very high speeds the ammunition is normally composed of a narrow penetrator surrounded by a sabot which expands the diameter to the full barrel width of the firing gun. This allows the pressure of the propellant gases to act on the full-size base and produce rapid acceleration of the round, which is lighter than a full metal round of the same diameter would be. Once the round leaves the barrel the sabot falls off, leaving the penetrator travelling at high speed and with a smaller cross-sectional area, which reduces aerodynamic drag during the flight to the target. This technique was first used in anti-tank guns during World War 2; Germany developed sabots under the name "Treibspiegel".

KE-penetrators for tanks are commonly just 2–3 centimeters in diameter, and 50–60 centimeters long; as more modern penetrators are developed, their length tends to increase and the diameter to decrease. To maximize the amount of kinetic energy released on the target, the penetrator must be made of a heavy material, such as tungsten or DU (depleted uranium). The hardness of the penetrator is of lesser importance. In fact uranium is not particularly hard. An advantage of uranium is that it is pyrophoric: the molten fragments of the penetrator ignite on contact with air. Uranium rod is also self-sharpening on impact.

Few countries use DU ammunition because of its cost and environmental effects. Battle sites where DU rounds have been used typically have residual uranium dust in and around battle-damaged vehicles. This dust is mildly radioactive and toxic, thus the lingering effects of it can create public relations problems.

Because a long, thin rod is aerodynamically unstable and tends to tumble in flight, two different approaches have been used to stabilise them. The oldest is rifling, which spins the round. An alternative approach is to add fins like those of an arrow to the base and fire the round from a smooth-bore gun. This is the approach commonly used in recent Russian, German and US guns. Sometimes a rifled barrel has been combined with fin stabilisation, using some system to prevent the round from spinning in the barrel. The rifled barrel approach improves the accuracy of the other types of ammunition which must be fired. An example of this is the L30 120 mm rifled tank gun used on the United Kingdom Challenger 2 MBT.

It is generally accepted that KE-penetrators are the most effective ammunition in penetrating armour today. The other main type of tank ammunition is High explosive anti-tank (more common in missiles). The British prefer HESH (High Explosive Squash Head).

The counterpart of a KE_penetrator in rifle ammunition is the flechette. A rifle firing flechettes, the Special Purpose Individual Weapon, was under development for the US Army, but the project was abandoned.