The present invention provides a subsonic ammunition including a polymeric casing body; a propellant insert positioned in the propellant chamber to reduce the internal volume of the propellant chamber; a propellant disposed and confined within the propellant chamber; a primer insert positioned at the body base and in communication with the propellant chamber; a primer and diffuser disposed in the primer insert in combustible communication with the propellant; and a projectile frictionally fitted in the mouth in combustible communication with the propellant.