thanks a lot Kosmos and Smurata1, I think your suggestions are close to my problem's solution, but I can't really be sure exactly how:
Sooo tonight I added a metal hop up.
the metal ring can't really fit between the hop up and the barrel; how critical is it exactly ? I stuffed it as deep as I could...
other than that, at the barrel level the seal seems ok...when I blow into it
I also changed the cylinder set for a systema one on the first mechbox.
The cylinder head is suprisingly loose in the cylinder.it is ventilated.
The spring is not reverted (linear end front,compressed nonlinear end towards shooter)
aaand I added a metal nozzle with the O-ring.
it still shoots 249 fps!
I checked the seal with the bucking when the barrel is mounted, it looks uber-perfect.
Could there be a leak at the junction mechbox-hop-up ?