A high strength polymer-based cartridge casing includes an upper component of polymer having a first end with a mouth, at least a wall between the first end and a second end of the upper component opposite the first end, a sleeve extending longitudinally and radially about the wall, and at least one of an overlap portion and an underskirt portion extending from the wall near the second end. The lower component is molded from a polymer and includes at least one of a tapered portion and an outer tapered portion that engages at least one of the overlap portion and the underskirt portions, respectively, to join the upper and the lower components. Further, the sleeve reduces a volume of a propellant chamber formed by the wall. The reduced volume of the propellant chamber permits only enough propellant to propel a bullet engaged in the cartridge casing at subsonic speeds.