(HEAT) high-explosive anti-tank round

A high-explosive anti-tank (HEAT) warhead is a type of munition made of an explosive shaped charge that employs the Munroe effect, to create a high velocity partial stream of metal in a state of superplasticity, which is used to penetrate solid vehicle armour. It is used in weapons such as the AT-4 single-shot rocket launcher, all the way up to the warheads of main battle tanks.

The HEAT warhead has become less effective against tanks and other armored vehicles due to the use of Chobham armor and explosive-reactive armor on systems such as the Russian T-90 main battle tank. Infantry weapons such as the US Army FGM-148 Javelin attack the top of tanks where armor is typically thinner than the front and side armor.
The stream moves at hypersonic speeds in solid material and therefore erodes exclusively in the contact area of jet and armor material. The correct detonation point of the warhead and spacing is critical for optimum penetration, for two reasons:

If the HEAT warhead is detonated too near a target's surface, there is not enough time for the particle stream to fully develop. That is why most modern HEAT warheads have what is called a standoff, in the form of an extended nose cap or probe in front of the warhead.[notes 1]
The distance is critical because the stream disintegrates and disperses after a short distance, usually well under 2 metres. The stream material is formed by a cone of metal foil lining, usually copper, though ductile iron and tin foil was commonly used during World War II.

The key to the effectiveness of a HEAT round is the diameter of the warhead. As the penetration continues through the armor, the width of the hole decreases leading to a characteristic fist to finger penetration, where the size of the eventual finger is based on the size of the original fist. In general, very early HEAT rounds could expect to penetrate armor of 150% to 250% of their diameters, and these numbers were typical of early weapons used during World War II. Since then, the penetration of HEAT rounds relative to projectile diameters has steadily increased as a result of improved liner material and metal jet performance. Some modern examples claim numbers as high as 700%