DanBow

You will find that with almost every shot, unless you wait a minute or so between them, the FPS will likely drop - due to the cooldown of the gas.

Another reason that it might be wildly fluctuating- especially if you are getting a few sky high readings  -is that the Chrono is picking up the gas cloud itself. In which case, try holding the muzzle a little way back from the chrono (whilst taking a lot of care not to shoot out your screen!)

Thats a good point, there were a few shots that registered as shots but no reading showed.  I presume that waa picking up the gas?

Possibly....worth trying a little back and seeing what happens!