High Explosive Squash Head
High Explosive, Squash Head (HESH) is a type of explosive ammunition designed to defeat tank armour. It was used by the Canadian Army after the Second World War, and used as a main battle tank ammunition, with versions in both 105mm (as used by the Centurion tank), and 76mm (as used by the Cougar).
HESH rounds contain a warhead filled with plastic explosive and a delayed fuse. On impact the plastic explosive in the shell spreads out to form a disk on the surface of the armour. The fuse then detonates the explosive, creating a shock wave which travels through the armour, causing flakes of metal to spall off the armour's inside surface. The resulting fragments injure or kill the crew, damage equipment, and/or ignite ammunition and fuel. Unlike HEAT ammunition, HESH shells are not explicitly designed to penetrate the armour of main battle tanks, although performance depends on the thickness of the target's armour plating.
HESH was developed by (Charles) Dennis Burney during the Second World War as an anti-fortification munition for use against concrete. However, HESH rounds were found to be effective against metal armour as well. HESH was used for some time as a competitor to the more common HEAT round in British units, including infantry units equipped with recoilless rifles.
HESH rounds were most effective against Soviet vehicles built in the 1950s and 1960, and their use declined from the 1970s onwards. Tank armour became more difficult to defeat, due to layered designs using composites of hard metal and heat-resistant materials that don't transmit shock well. As well, a switch to smooth bore cannon as a major trend in ordnance construction has made HESH rounds less useful, as they rely on spin for accuracy.