If you did a live fire zero, then most likely the barrel is installed crooked, or barrel vibration is throwing your shots off of bore sight, or both. I see these issues often in production rifles. Crooked base installation seems unlikely.
You can diagnose what's out of alignment by using an 18" metal ruler. Place the ruler along one side of the receiver with 12" or so extending beyond the barrel nut. Holding the ruler flat against the receiver, measure the distance between the end of the ruler and the barrel. Now repeat this measurement on the other side of the receiver. If the two measurements are not equal, the barrel is installed crooked. You can then calculate the angular offset.
Be sure to buy Burris Signature Zee rings. You will probably need to use offset inserts on the from and back rings to correct the misalignment.
Regardless of how the scope is installed, you won't have a canting problem if you use an anti-cant indicator that is accurately aligned to the turret axes, You may have a range dependent aiming error, which you can eliminate using the Reticle Alignment Tool that HighPowerOptics sells. That tool also has a very accurate spirit level.
Last edited by bruce_ventura; 10-18-2013 at 08:43 AM..