A tricky problem!

[angrybear]

Is it possible for the breach block to contact the pellet skirt & deform it ?

Can you speak to the person who did the work ? he might remember the rifle having an issue from the off.

[crowbar]

I would strip the guts out clean the bore and push a pellet through it and see what I can find .
Even at 20 fps variation impact point should be negligible , shine a light down the bore and cotton bud the crown .

[Gary C]

Barrel imperfection taking a chunk out of the pellet, or re-crown. Damaging a crown only takes a pull through

[Shed tuner]

for it to be that bad...

with the rifle stripped, try pushing a pellet through it very carefully, see if you feel anything odd...

[Rapidnick]

Thanks to everyone who has contributed. I'll revert after further testing.

[RichardH]

Nick

It’s the crown, barrel or pilot error in that order.

If it’s a .22 bring it to Kempton and I will recrown it while you wait, it’s a five minute job with the right tool.

Rich

[Gary C]

Nick

It’s the crown, barrel or pilot error in that order.

If it’s a .22 bring it to Kempton and I will recrown it while you wait, it’s a five minute job with the right tool.

Rich

...and you are a right tool !

[RichardH]

...and you are a right tool !

Takes one to spot one old sport

[averageplinker]

Nick

It’s the crown, barrel or pilot error in that order.

If it’s a .22 bring it to Kempton and I will recrown it while you wait, it’s a five minute job with the right tool.

Rich

I think we can rule out pilot error. I have seen Nick shoot and he can do better than the 9" groups he reports with this rifle!
😀

[Rapidnick]

Nick

It’s the crown, barrel or pilot error in that order.

If it’s a .22 bring it to Kempton and I will recrown it while you wait, it’s a five minute job with the right tool.

Rich

Thanks for the offer Rich but it is .177.

[angrybear]

Have you tried shooting some pellets in to water so that you can look for any type of damage caused during shooting ?
might give you a better idea of where to start looking.