New Chrono battery, and took the spring and guides out of the old leather sealed gun that was doing 730fps, and put it in the "new" synthetic sealed Hw50.
This spring is a bit shorter, but has thicker wire (3,1mm) and smaller ID at about 13,5mm.
Th gun is still sealed with JM/ARHs 25mm HW seal, that I have sized a bit.
Recoil is very soft, and there is no vibration. Seems like the shorter stiffer spring gave a bit softer recoil.
I might get a few more fps by changing the seal or sizing it a tad more, but I'm afraid this might make the gun less pleasant to fire.
10 shots over the chrono gave an average of 680fps with 7,9 JSB, and a maximum spread of 8fps.

I'm happy for now, but would like a bit more velocity... Its time to shoot some targets and see how the gun runs in before any more changes are made.