My friend has a Christensen Arms Steel Classic rifle. The top of the action has milled slots to accept weaver style rings. He has a Vortex Viper PST 6-24x50 30mm tube scope with Warne split rings medium height. The problem is once he has it mounted and bore sighted he is left with about 12 minutes of elevation. He would like to practice out to 1,000 but 12 minutes won’t get him that far. What’s next? Switch rings? Could he shim the rear ring? But I’m guessing that would cause issues, unless he lapped the rings. Is it the scope?