Hello Jeff,

I too would be flummoxed after having gone through the checks you have.

Are you still using the same tin of R10s? or a different tin?
Has the grouping also suffered with the other pellets you said you had previously been happy with?
As unpalatable as it may be, is there anything the shooter is doing that introduced the expansion of the groups?

Clearly something has changed from when you were able to get the fantastic group. You quoted Einstein (with which I totally agree), so to quote someone else "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.". You now have to think of the improbables, which will include everything not just the mechanics of the gun. The checklist will be vast, good luck.

ps different guns for different people - I sold my HW100 and kept my Mk4is.