RE: Sighting In Problem....
Is your groups walking? Meaning string out after the first shot as the barrel heats up. My browning SS Stalker did this out of the box, a simple fix just used a dremel and hawged out the areas of the barrel channel in the forearm of the stock where it put pressure. To test just run a sticky note along side the barrel and stock to check for tight areas. It should run freely. If this is good being you've tried several ammo choices I'd befollowing the others advise about themounts and scope. Check all the screws to ensure they are tight, when putting the base screws in I count the revolutions until it bottoms before installing the bases, then when I place on the bases I cout again to ensure the base screw is not bottoming out before holding the bases snug. If this checks out I use the blue loctitte and install.
If you have a spare scope try installing it to see how it shoots in comparison, this should either eliminate the mounts or scope.
Good Luck