The power spring in an airsoft gun is a mechanism for storing energy. When the spring is released, that energy is expended in overcoming whatever is preventing the spring returning to its pre-compressed length, which can be friction in the guide rails or between the piston head O ring and the cylinder, or air resistance. Provided that the spring is always compressed to the same length, the energy stored will always be the same, regardless of how that compression is achieved, assuming that the spring strength remains constant..