The Infinitengines Company has been developing their "Dreamair" pneumatic speargun for many years. It is basically a high speed winching system where compressed air in the gun body is further compressed when you cock the gun by moving a large piston. Unlike nearly every other pneumatic speargun the piston is dragged forwards in the gun when you cock it and when the gun shoots the piston travels rearwards. The reason for this is the piston is attached to a cable that winds on and off an inner axle inside the pressurized spaces of the gun. On the exterior of the gun and mounted either side of the same axle are winding drums that that are turned and haul the long cable wishbones in that pull on the shaft. So externally it looks much like a band gun, but instead of rubber bands the cables wind up on the external grooved drums when you shoot and when you pull the cable back to cock the gun that action rotates the drums and axle and a co-axial inner drum that via an inner cable hauls the internal piston forwards thereby recharging the gun for the next shot.
Recently a Carbon version of the "Dreamair" was announced, the first version was an alloy gun that has never been marketed as a couple of years ago the company suddenly decided that more divers would be interested in a carbon fiber version of the gun.https://www.youtube.com/watch?time_cont ... ZnqMrFt7ys