An explosively formed penetrator (EFP) – also known as an explosively formed projectile, a self-forging warhead, or a self-forging fragment – is a special type of shaped charge relying on the Misnay–Schardin effect, designed to penetrate armour effectively, from a much greater standoff range than standard shaped charges, which are more limited by standoff distance. As the name suggests, the effect of the explosive charge is to deform a metal plate into a slug or rod shape and accelerate it toward a target. They were first developed as oil well perforators by American oil companies in the 1930s, and were deployed as weapons in World War II.