page_title: Armor-piercing shell
section_title: High-explosive squash-head or high-explosive plastic
hierarchical_section_title: Armor-piercing shell / Types / Armor-piercing shells / High-explosive squash-head or high-explosive plastic
caption_attribution_description: English: The 29cm Petard spigot mortar on a Churchill AVRE of 79th Squadron, 5th Assault Regiment, Royal Engineers, under command of 3rd Infantry Division, 29 April 1944. A 40lb bomb can be seen on the right. The 29cm Petard spigot mortar on a Churchill AVRE of 79th Squadron, 5th Assault Regiment, Royal Engineers, under command of 3rd Infantry Division, 29 April 1944. A 40lb bomb can be seen on the right.
context_page_description: An armor-piercing shell, armour-piercing shell in Commonwealth English, AP for short, is a type of ammunition designed to penetrate armor. From the 1860s to 1950s, a major application of armor-piercing projectiles was to defeat the thick armor carried on many warships and cause damage to the lightly-armored interior. From the 1920s onwards, armor-piercing weapons were required for anti-tank missions. AP rounds smaller than 20 mm are typically known as "armor-piercing ammunition", and are intended for lightly-armored targets such as body armor, bulletproof glass and light armored vehicles. The AP shell is now seldom used in naval warfare, as modern warships have little or no armor protection. In the anti-tank role, as tank armor improved during World War II newer designs began to use a smaller but dense penetrating body within a larger shell. These lightweight shells were fired at very high muzzle velocity and retained that speed and the associated penetrating power over longer distances.
  An armor-piercing shell must withstand the shock of punching through armor plating. Shells designed for this purpose have a greatly strengthened body with a specially hardened and shaped nose.
context_section_description: High-explosive, squash-head (HESH) is another shell based on the use of explosive. It was developed by Charles Dennistoun Burney in the 1940s for the British war effort, originally as an anti-fortification "wallbuster" munition for use against concrete. Despite this, HESH was found to be surprisingly effective against metallic armour as well.  
  HESH rounds were thin metal shells filled with plastic explosive and a delayed-action base fuze. On impact, the plastic explosive is "squashed" against the surface of the target, spreading out to form a disc or "pat" of explosive. The base fuze detonates the explosive milliseconds later, creating a shock wave that, owing to its large surface area and direct contact with the target, is transmitted through the material. At the point where the compression and tension waves intersect a high-stress zone is created in the metal, breaking off a "scab" of steel. This, in addition to smaller spall, is projected off the interior wall at high velocity, damaging the equipment and crew without actually penetrating the armour. 
  Unlike high-explosive anti-tank (HEAT) rounds, which are shaped charge ammunition, HESH shells are not specifically designed to perforate the armour of main battle tanks. For this, the British already had effective weapons using HEAT, such as the PIAT. HESH shells instead rely on the transmission of the shock wave through the solid steel armour. Thus, HESH is defeated by spaced armour, so long as the plates are individually able to withstand the explosion. It is, however, still considered useful, as not all vehicles are equipped with spaced armour, and it is also the most effective munition for demolishing brick and concrete.
  HESH was for some time a competitor to the more common HEAT round, again in combination with recoilless rifles as infantry weapons and was effective against tanks such as the T-55 and T-62. HESH shells, unlike HEAT shells, can be fired from rifled guns as they are unaffected by spin. In American usage it is known as high-explosive plastic (HEP).
caption: Petard spigot mortar launcher and 290mm HESH round, on Churchill AVRE
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