High Explosive Squash Head (HESH), for which the American term is High Explosive Plastic or HEP, is a type of chemical energy ammunition which is not velocity dependent. In this round the explosive is contained in a thin-walled projectile which collapses on striking the target, allowing the plastic explosive to spread. A base fuze then denotes the explosive which sends strong shock waves through the armour. Reflection from the internal armour surface causes an overmatch of the armour which then fails, causing a large 'scab' to form and fragments to fly off inside the vehicle. The success of HESH against armour depends on the explosive forming a suitably shaped 'pat' on the outside of the armour plate before it is detonated (the British term 'squash-head' describes the process very well). This poses several problems, because if the projectile hits the target at a very acute angle, the explosive will not form a cohesive mass and consequently its effectiveness will be greatly reduced. Similarly, if the projectile arrives at the target with very high residual velocity, i.e. if the target is at very close range, the explosive will be dispersed on impact, before the fuze has time to function. The latter problem is easily resolved by keeping the muzzle velocity low, but the result is a lower chance of a hit because of the higher trajectory. HESH can be defeated relatively easily by having a discontinuity (spaced armour) in the path of the shock wave, though the outer plate will almost certainly be destroyed.

The major advantage of HESH lies in its usefulness as a multipurpose round. It is approximately 90% as effective as a conventional High Explosive (HE) round against unarmoured targets and considerably better than HE against bunkers and buildings. It has a devastating effect against lightly armoured targets and even if a kill is not obtained against a more heavily armoured target, the secondary effects will damage all the optics, antennae and any equipment mounted externally.