LAST EDITED ON Dec-12-12 AT 06:10PM (MST)[p]Do you have another rifle or scope that you "KNOW" is on or works well? You could switch your "scope in question" to that known good shooter to verify it still holds true, or you can mount the "known good scope" to your rifle in question and see if the groups get better.
With that said, I would make certain that you have thoroughly cleaned the rifle of excessive copper. Have you run something like CR-10 or Butch's Bore Shine or Sweets through your barrel? Sometimes, you will be surprised how much copper can build up in your barrel. Also, have you changed loads or components? That can make a huge difference too. As a handloader, I am still amazed at the difference half a grain of powder can make with groups.
Also, check the parralax on your scope. With the rifle firmly placed in a vice or similar, look through to your target at 100 yards. Then, with your face not touching the stock, move your face around a little bit and see what happens to your crosshairs. If you are not touching the stock, it shouldn't move more than half an inch to an inch at 100 yards. I have had "good" scopes show parralax in excess of 3-4" at 100 yards. That can make a huge difference in your group size. If you have that kind of Paralax, then send it in to be reset. I have read really good things about Sightron and their Customer Service too.
I have learned a lot about the parralax issue by running the "zero matic" machine for a local gun range. Sometimes that is an eye opening experience, but I have learned a lot. Both about scopes and rifles, and.....people.
I have a Weaver V-9 that has held its zero on EVERY rifle I have mounted it on. I have had terrific luck with my Burris Signature Select and a Burris Fullfield II. That said, I have seen great luck with really cheap scopes holding zero and really expensive scopes not holding zero. Even the best can go bad on occasion. I once owned a Redfield Tracker that was mounted on my Win 70 in 7mm Rem Mag. Parralax was huge on that scope. I sent it in for repair and they were able to fix it. I sold that scope to a buddy as I had to buy the Weaver while it was in being repaired. He has used it now for years on a Model 700 in .30-06 and it is still good to go.
Let us know what you find out.
FH