High Explosive Incendiary/Armor Piercing Ammunition (HEIAP) is a form of shell which combines both an armor-piercing capability and a high-explosive effect. In this respect it is a modern version of armor piercing shell. The ammunition may also be called Semi-armor-piercing high-explosive incendiary (SAPHEI).
Typical of a modern HEIAP shell is the Raufoss Mk 211 .50 BMG round designed for weapons such as heavy machine guns and anti-materiel rifles.
The primary purpose of these munitions is armor penetration, but unlike SLAP rounds (Saboted Light Armor Penetrator) which get their armor-piercing ability from the propulsion of a 7.62mm tungsten heavy alloy bullet from a 12.7mm barrel (.50 caliber) using a sabot with much more energy than is usually possible from a 7.62mm round, HEIAP munitions use high explosives to "blast a path" for the penetrator. The special effect is developed when the round strikes the target. The initial collision ignites the incendiary material in the tip, triggering the detonation of the HE charge. The second (zirconium powder) incendiary charge will also ignite. This burns at a very high temperature, is not easily extinguished, and can last for 15 minutes
The remaining element of the round is the tungsten carbide penetrator. This has a large amount of kinetic energy and will penetrate the armor as a solid-cored armor-piercing shot would. This will take some of the incendiary material through the armor. The MK 211 is claimed to penetrate up to two inches (51 mm) of rolled homogeneous armor.
The triggering of the explosive charge is dependent upon the resistance of the target. If the target offers little resistance then the lack of frictional heating will prevent the incendiary from igniting and the high explosive from detonating.
Larger guns such as the British 30 mm RARDEN cannon fire APSE (Armour Piercing Special Effects) shells which are an armor-piercing round with added HE effect.
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