MAJOR loss of FPS

Hi Guys, new to the forum, done a quick search but found no results on Poor FPS so apologies if this has already been answered.

Im the proud owner of a JG bar 10, it has resently been upgraded to know contain:

Laylax zero trigger and Piston
Laylax Spring guide
Laylax 170 spring
509mm 6.03mm black python Tight bore barrell

I have PTFE'd the cylinder head and the hopup rubber.

Now it should be shooting at around 550fps. But using the poor mans Chrono, it's not even passing through the bottom centre of the can, now i think that means it must only be firing at around 385fps (ruff guess unless someone can confirm).

So my question is.... What the F@%ks gone wrong, where has all the fps gone. Have i put the spring in the right way am i a complete smeg head(don't answer that one, the wife's already confirmed that on many occasion!!). Please help im starting to get depressed and i might have to start listening to REM.

What weight of BB's are you using?

Im testing the fps out on .2g BB's currently. Wanting to go up to poss .4g BB's if i can get the FPS sorted.