Just a further thought on producing a standard test. We have seen in the past that as soon as you put in place legislation to control something, people get very creative to work around it.
If someone does come up with a consistent , fair, repeatable test for airgun power, you can bet someone out there will find a way to exploit it. That is to say, produce a rifle capable of passing the test, but which may under special circumstances deliver more power than what is permitted. Perhaps this is why there is no standard test.