I had a similar challenge with a .270 Winchester a few years ago. My groups with the scope were over six inches at 100 yards and the group centers varied by more than that from session to session. The see-through base let me use the factory open sights. I got something under 1.5" with a three-shot group with them, so I knew something was wrong.
I checked the tightness of the scope mounting screws -- they were tight.
I checked the tightness of the screws holding a the base to the action -- they were tight.
The remaining possible cause was a defective scope. Swapped scopes and the groups tightened to better than the iron sights and the zero stayed.the same between range sessions, so I became a happy camper.
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