The gas strut has less pressure in it and they either use a longer piston to reduce the cocking stroke or drill a hole in the piston face- they have made sure there is almost no way one could ever go over 500fps no matter what the malfunction may be (paranoia about the Canadian law it seems)- so they have used two detune methods on them (which also results in them tending to shoot closer to 440fps out of the box when new, not 495fps).