You don't say which 'mk' of Airsporter it is......this will determine your answer.
If it is a mk1 or mk2 then you can buy taller front sight blades.
If a late mk5 or 6 the front sight blade can be raised in its ramp. If the latter....slacken the screw that fixes the front sight to the barrel and pull up on the actual foresight blade. Once you have raised it tighten the screw and re test.
You can raise or lower the blade until you have the correct result.
If it shoots high you need a taller front sight or raise the blade if it is a later model.
Pete.