I fitted a new piston and breech seal today, removed 1 1/2 coils off the spring and it did a ten shot string: 572-576 fos, average 11.67 ft/lb.
It is super smooth to shoot and seems very accurate at 50m (small flower heads). More testing will follow on a calmer day, but I think it's yet another keeper.
One little thing....... What's the point of the thumb catch? Unless it is adjustable it seems to serve no purpoe as the barrel slips past it easily.