Abstract: The present invention provides a subsonic ammunition comprising: a primer insert comprises a top surface opposite a bottom surface, a substantially cylindrical coupling element extending away from the bottom surface forming, a primer recess in the top surface that extends toward the bottom surface, a primer flash aperture positioned in the primer recess to extend through the bottom surface, a flash aperture groove in the primer recess that extends circumferentially about the primer flash aperture, and a flange that extends circumferentially about an outer edge of the top surface; a polymeric casing body comprising a generally cylindrical hollow polymer body molded over the substantially cylindrical coupling element, into the primer flash aperture and into the flash aperture groove to form a flash hole and extending toward a mid-body coupling joint; a nose comprising a projectile aperture that extends to a neck connected to a shoulder that transitions from the projectile aperture to a nose coupler, wherein the