The error in using any kind of spring gun is that an increase of pressure acts against the pellet and the piston to retard ,or actually drive back the piston......thereby acting as a sort of safety valve..................so ,for the system to work ,at least you would need some kind of double diameter piston setup,so that the increased pressure due to fuel ignition did not act on the full area of the main piston,but on a reduced area of the smaller piston.....and as mentioned this makes the gun a firearm,and restricts sales ,whereas airguns can be sold freely in most parts of the world............If firearms are OK,then why not a much cheaper .22 rimfire.