This concerns what on the face of it looks an excellent rifle. It is an early 25mm HW77 which has has a new spring, spring guide and top hat installed by a very experienced tuner with an excellent reputation at my club. Shooting a 10 shot string over my chrono gave a reasonable-although in fairness not fantastic- consistency showing a MV between 734 and 753 ft sec with a large cluster between 746 and 753 fps.. Honestly I don't suspect the power plant because in my view the variation in MV does not explain the 9" group of 10 shots over 30 metres which this rifle produced in a random spread during a wind free spell earlier today. In addition the group ranged from left to right as well as up and down.
I have taken the gun apart and checked the following:
The piston seal-excellent condition. looks unworn.
The spring-has polished ends and the guide and top hap fit perfectly.
(Really for the sake of it I have changed the spring/guide/top hat and the results are the same.) I really do not suspect the power plant.
The scope-changed several times, no improvement.

What can be wrong? Before you answer, please bear in mind that I am not clueless on spring rifles and whilst not a great shot, I do understand how springers work and usually can shoot them pretty well. In any event a 9" group is well below my usual standard even on a bad day!
Thanks for reading.