Not good to make a pellet gun diesel by using something flammable/explosive. The guns aren't made for mini explosion's. When pellets go supersonic, they can float way of course.
I remember reading, some 30 or so years ago one of the German makers actually made a diesel gun, a spring air model that had a tiny injector that injected some ether in the cylinder to ignite and create more pressure and felocity. If I remember correctly it created enough pressure that it actually re cocked the piston upon firing.
