The present invention provides an ammunition cartridge having a primer insert having a top surface opposite a bottom surface and a extraction flange that extends circumferentially about an outer edge of the top surface; a coupling element that extends from the bottom surface, wherein the substantially cylindrical coupling element is adapted to receive a polymer overmolding; a primer recess in the top surface that extends toward the bottom surface, wherein the primer recess comprises a recess bottom and a circular recess side wall; a primer flash aperture through the recess bottom that extends through the bottom surface, wherein the primer flash aperture is adapted to receive a polymer overmolding to form a flash hole; and a groove in the primer recess positioned around the primer flash aperture adapted to receive a polymer overmolding, wherein the groove extends at least partially over the bottom surface and at least partially over the circular recess side wall and the groove is adapted to receive polymer over the bottom surface and at least partially over the circular recess side wall.