In warfare, high-explosive incendiary (HEI) is a type of ammunition specially designed to impart energy and therefore damage to its target in one or both of two ways: via a high-explosive charge and/or via its incendiary (fire-causing) effects. Each round has both capabilities.
HEI ammunition is fused either mechanically or chemically. The armor-piercing ability can vary widely, allowing for more focused fragmentation or larger scatter.