You have to change the concept of how the test runs.
If you have a "standard" pellet for use over the chrono then the rifle mustn't be over with that pellet. If it happens to be shooting at a higher level with it's own pellet then that has to be discounted.
If the test pellet runs at 780 but your pellet runs at 799 then that has to be accepted. The "test" pellets would also have to be available so owners could check that their rifle performs as expected at the chrono.
A different concept to what is done now but as legally there isn't a test, and legally the rifle must not be capable the current system hardly absolves anyone on a legal basis.