In three colours for US$119.50 at Airsoft Extreme... "The patent-pending Magpul® ACS™ (Adaptable Carbine/Storage – Military Model) is a drop-in replacement butt-stock for carbines using mil-spec receiver extension tubes. A streamlined, anti-snag profile shields the release latch to prevent accidental activation and the extra-power lock mechanism uses a premium chrome-silicon spring to avert auto-collapse failure by resisting spring relaxation.