From Sots2

This modified armor-piercing design that adds a payload of incendiary, high explosives. The high explosives are activated upon impact, aiding the entry of the armor-piercing core, which in turn draws high explosives through the hull, together causing far more damage than a standard AP round. Activation of the HE component is critical, meaning the greater resistance a target's armor provides, the more the high explosive is to be activated. Allows for the HEAP variant in light, medium and large barrelled rounds.

The research of HEAP Rounds opens the possibility to research the following technologies: