A high strength polymer-based cartridge casing includes an upper component of polymer, a bullet of a standard weight, a lower component of polymer, and an insert. The upper component has a shoulder portion and an upper component inner wall has a first slope extending from the shoulder. The lower component has a lower component inner wall having a second slope. The upper and lower component inner walls form a propellant chamber; and the first and second slopes reduce a volume of the propellant chamber. The reduced volume of the propellant chamber permits only enough propellant to propel a bullet engaged in the cartridge casing at subsonic speeds.