High explosive squash head

High explosive squash head, also known as HESH or HEP (in US usage, for high-explosive, plastic), rounds are a type of anti-tank explosive. On impact the plastic explosive in the shell spreads out to form a disk on the surface of the armor, which is then detonated. The explosive does not penetrate the armor directly, instead the shock wave travels through the armor and causes flakes of metal to spall off and fly around the interior of the tank. The resulting fragments injure or kill the crew, damage equipment, and/or ignite ammunition and fuel. HESH shells will rarely actually penetrate the armor.

Developed and used only by the British originally as an anti-pillbox munition for spalling concrete, HESH was found to be surprisingly effective against armor as well. HESH was used for some time as a competitor to the more common HEAT round, and is effective against tanks from the 1950s and 1960s such as the T-55 and T-62.

Against modern armor HESH is basically useless. Modern tanks have composite or layered armor that does not transmit the shock wave very well, and spall liners made of materials such as Kevlar to catch any flaking that does occur.

HESH rounds are still used by the British Scorpion tank and some other AFVs, but this is more for its use against fortifications than other AFVs.