I had same problem. Even tried some Warne rings, didn't help.
Although I can't prove it, it appears to me the action cut for the barrel is not true to the center line of the action. Th gun was an inherited 2003 model, so I opted not to send back to Ruger.
My fix was a folded portion of beer can on sides of the mount. For me, it was shooting left, so I shimmed the left rear and right front base.
Base adapters (to weaver) and Burris rings were an option but I read issues/negatives with just about every solution. So far my beer can has worked.