Have you tried firing from a sturdy rest to minimize human error? It may help with the diagnosis.
Pellet rifles are very sensitive to "follow through". Any movement at the time of firing is magnified.
Some designs are more sensitive than others. My Webley Tempest pistol shoots terrible groups if it is rested on a hard surface when firing. Some padding on a rest usually helps.
Yes, I have fired from a rest.
It did slightly improve the groups.
Pellet rifles are ‘sensitive’.
Googling - “But, like almost all high power (1000 fps and above) spring/piston air rifles – and gas ram guns for that matter too – the Whisper requires skill and experience from the shooter for accurate shooting. Use of the loose “Artilley Hold” is mandatory and a tight grasp on the gun will destroy the accuracy of which it is capable.”
I would guess the large group size is due to shooting technique.
Should shrink with ‘proper’ (whatever that is) technique.
Then should shrink from that with the pellet it likes.