Webley Osprey ST weird flyers

[hwtyger]

Always had some issues with 1 of my Osprey STs.
First shots sometimes are way off. Like 1 or 2 cm at 6 yrds. I know I dont shoot full score each card but thát far off at so close a distance...
Theyre not off in the same direction.
Im still looking for the cause.
Its either me, cause I do find the Osprey to be very hold sensitive or its an issue I havent thought of yet. If its me you'd expect me to have it with my other rifles as well,(MK3 ST and Walther LG55) which I dont. My other Osprey ST also doesnt seem to have this issue.
Im using regular target stance; top of hand underneath the stock. Thumbup shootinghand position. Tried and tested various variations, but this seems most accurate.

What its nót:
- all bolts are loctited so nothing is loose
- no barrelcrown issue; as after a few shots it seems to settle and I regularly dó shoot full score or amazing groups with it
- nothing is worn internally as its all new
- tried cleaning barrel, made no difference (wasnt dirty anyway)
- theres no smoke, slowburn or dieseling
In case of tap alignment, youd expect issues all the time.
Its a dream to shoot.

I use JSB Exact 4.51 cause I use it in all. Its easiest and it seems to do ok in it.
I shoot indoors, so temperatures never go below 18.5C in coldest winter. They may rise in the heat of summer. But few days ago, temperatures were nothing special and stíll shots way off

Could it be a piston seal thats either too tight or too sloppy that needs a few warming up shots?

Other ideas?