A pre-charged pneumatic air gun includes a barrel, a firing valve, a trigger assembly, a compressed gas storage tank, a first chamber, a larger second chamber and a compression piston movable within the first chamber and the second chamber. The compression piston is operable to increase a pressure in the first chamber to a final first chamber pressure greater than an initial first chamber pressure by sequentially opening a first fill valve with the second fill valve closed to allow compressed gas to flow into the first chamber then opening a second fill valve to allow the compressed gas to flow into the second chamber, thereby displacing the compression piston and further compressing the compressed gas in the first chamber to achieve the final first chamber pressure.