A tricky problem!

[Rapidnick]

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.

[si7777777]

What does the bore and crown look like?

[landymick]

Was it OK before the new parts ?

Check the obvious like stock screws & breach seal first.

[Rapidnick]

Was it OK before the new parts ?

Check the obvious like stock screws & breach seal first.

Honestly I don't know as the rifle came to me with them fitted. However what would have to be the problem with what looks like a well finished spring and properly fitted guides to give me such terrible groups?
Stock screws properly tightened and breech seal looks fine.

[Weasle]

Honestly I don't know as the rifle came to me with them fitted. However what would have to be the problem with what looks like a well finished spring and properly fitted guides to give me such terrible groups?
Stock screws properly tightened and breech seal looks fine.

Hi Nick

Just to narrow the fault down - is it the same shotgun spread with a selection of pellets - makes and head size?

Cheers Steve

[MattyBoy]

There must be an issue inside the barrel which is forming the pellets into a lopsided shape.

One little rust spot could do this I guess?

Matty

[Rapidnick]

Hi Nick

Just to narrow the fault down - is it the same shotgun spread with a selection of pellets - makes and head size?

Cheers Steve

We have spoken. I think you are on the right track-recrowning is probably the answer.

[Rapidnick]

What does the bore and crown look like?

Crown looks fine. Bore not easy to check as it is a fixed barrel rifle. Next job to do.