that sounds about right. But with that 300mm he could use heavier bbs and net some nice results(A 500mm barrel for 0.36gm) for I would I would expect about 500-510fps. .
Do what 1tonne said and re crono it dialed in for .40 or heavier . If you get the same results or LOSE power, then you are looking at an airleak. And since I didnt see a new cylinder or cylinder head in your list im going to guess thats where the issue is. ML buckings seem not to like stock cylinder heads
Now something you should know, that short barrel is going to cause creep a bit, so don't panic over lower .20 readings. Use joules and watch the deviation of the FPS readings to diagnose if its leaking somewhere