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 (think of throwing a large glob of wet modeling clay very forcefully against a brick wall), 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.

The British created antitank guided missiles in the 1960s using HESH warheads. It seems likely that they would have been highly effective, at the time.

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. There is some minor controversy in certain circles about whether HESH would be more effective against reactive armor tiles than other munitions; whether it would be depends on whether it is more likely that the explosives in the tiles would add to the blast and shock effect, or whether current reactive armor tiles incorporate shaped charges to focus the blast outwards.

HESH rounds are still carried today but this is more for use against fortifications than other armored fighting vehicles. A 165mm HESH round is used by the US Army for the main gun of the M728 Combat Engineer Vehicle, which is a sort of tank/bulldozer hybrid; the HESH round is intended mainly for use against concrete bunkers and similar structures. The Stryker Mobile Gun System variant is to be equipped with a 105 mm HESH round for the same purpose.

They are often called hash rounds because it is more natural to say than hesh.

See also: plastic explosive, Misznay-Schardin effect, Monroe effect, shaped charge