High explosive squash head
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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, and is then detonated. The resultant 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 by the British originally as an anti-fortification "Wallbuster" munition for spalling concrete, HESH was found to be surprisingly effective against armor as well. Other techniques for reducing the German defences had been found and it was considered for equipping artillery as an anti-tank round, but current shells were still effective for this purpose. 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 anti-tank guided missiles in the 1960s using HESH warheads (see Malkara missile). 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 armour 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 armoured fighting vehicles. A 165mm HESH round is used by the United States 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 Centurion AVRE (Armoured Vehicle Royal Engineers) is similarly equipped with a short 165mm gun solely for a 29 kg HESH shell.
The Stryker Mobile Gun System variant is to be equipped with a 105 mm HESH round for the same purpose. Argentina uses the 105mm HESH round to equip its TAM medium tanks.
They are often called hash rounds because it is more natural to say than hesh.