High-explosive squash head

HESH rounds are thin metal shells filled with plastic explosive and a delayed-action fuze at the base of the shell.[3] On impact, the inert material, followed by plastic explosive, is "squashed" against the surface of the target and spreads out to form a disc or "pat" of explosive. The inert material helps prevent premature detonation of the plastic explosive and sustains the impact pressure and temperature.[3][2]

Milliseconds later, the base fuze detonates the explosive, creating a shock wave that, owing to its large surface area and direct contact with the target, is transmitted through the material. In the metal armour of a tank, the compression shock wave is conducted through the armour to the point where it reaches the metal-air interface (the hollow crew compartment), where some of the energy is reflected as a tension wave, a phenomenon called impulsive loading. At the point where the compression and tension waves intersect, a high-stress zone is created in the metal, causing pieces of steel to be projected off the interior wall at high velocity.[2]

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  Shell design: charge, fuze, shell
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  Function of HESH-munition

This fragmentation by blast wave is known as 'scabbing' or 'spalling', with the fragments termed 'scabs or 'spall'.[4][2]

Depending upon the armour thickness, a heavy piece of target material (4 to 10 kg for a 120 mm round used in Arjun MBT[4]) can separate out from the other end of the target with supersonic velocities. This spall is sufficient to permanently damage the essentials of a tank, igniting the ammunition or fuel storage and severely damaging the crew to achieve a "total kill" of the target. In general, the higher the armour thickness, the higher the scab weight will be.[2]

The fragmentation achieved by impulsive loading of armour block by a HESH round is more lethal than are similar high explosive rounds.[4][2]

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  Front and back side of armour block impacted by HESH rounds
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  Light spalling damage caused by HESH rounds
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  Heavy spalling damage caused by HESH rounds

HESH rounds are mostly fired from guns with rifled, rather than smoothbored, barrels. Rifling causes a projectile to spin, creating centrifugal force and improving accuracy. This makes a HESH shell more effective on impact by increasing the surface area of contact for the explosive: the faster the spin, the larger the contact patch will be. Contrarily, the centrifugal force resulting from rifling would decrease the penetrating power of a high-explosive anti-tank (HEAT) shaped charge warhead by tending to disperse the shaped charge jet. HESH shells are not specifically designed to perforate the armour of vehicles, unlike HEAT rounds, with their shaped charge jets. HESH shells rely instead on transmitting a shock wave through solid steel armour.[2][5]

HESH ammunition has good general purpose use, being effective against most targets, though the round is generally used at relatively low velocities because high velocity excessively disperses the pat of explosive. While only effective against tanks without spaced armour or spall liners, the round is still favoured for combat demolition purposes. The flattened high-velocity explosive pat can destroy concrete constructions much faster than a HEAT round (which is designed to penetrate armour), and without the dangerous fragmentation of a traditional high explosive (HE) fragmentation round.[3]