I've had several that require shimming, and I put the shim under the base.
http://www.leupold.com/hunting-and-shooting/resources/faq/#questionSix6. My scope is running out of internal adjustment travel before I get properly sighted in. How can I obtain more travel?
When encountering an issue involving exhausted adjustment travel it is likely related to the alignment between the scope and barrel. When producing a firearm, there are many different components, each having a tolerance specification. As these tolerances “stack,” the alignment between the receiver and the barrel changes; this is why 10 seemingly identical rifles will all require different amounts of scope adjustment to sight-in. This is also why some scopes will reach the end of the adjustment travel without properly aligning to the bore; running out of adjustment before you can place the bullet in the center of the target.
This issue can be rather frustrating to the average rifleman who simply wants to sight-in and leave the adjustments in a single position, but to the long-range shooter who makes adjustments more often, the issue is compounded. People often want to know “how far can I shoot” with a particular scope, meaning how much elevation adjustment they will have for long-range shooting. This is not a question that can easily be answered because of the previously mention tolerance stack. As an example, let’s take 2 of the same 10 seemingly identical rifles mentioned earlier and see what happens when used for long-range shooting; both rifles are chambered in .308 Winchester, will be shooting 168 grain match-grade ammunition, and are mounted with scopes with having MOA of total adjustment travel (35-MOA up and 35-MOA down from center).
Due to differing tolerances, rifle #1 requires 10-MOA of down adjustment from the scope to be sighted in at 100 yards and rifle #2 requires 10-MOA of up adjustment. This means that the scope on rifle #1 will have 45-MOA of up adjustment remaining, allowing the shooter to make the proper correction for shots up to 1,070 yards; rifle #2 will have 25-MOA of up adjustment remaining, allowing the shooter to make the proper correction for shots up to 780 yards. Even though the rifles seem exactly the same, #2 will require a long range base or shims much sooner than rifle #1.
Elevation issues can be resolved by shimming (we will send shims free of charge if you would like). If more up adjustment is required, the rear of the base needs to be shimmed between the receiver and the base. If you need more down adjustment, the front base needs to be shimmed. In making this adjustment it should be noted each 0.001" thickness of shim equates to approximately 1-MOA (1 inch at 100 yards) correction. Shimming does not induce stress on the scope, but typically reduces stress by properly leveling the scope to be parallel with the receiver.
If an issue exists on the windage axis, the correction needs to be made with windage adjustable bases or rings. Leupold offers windage adjustable bases (STD) which have two windage screws holding the rear ring. By loosening one side and tightening the other, they shift the rear of the scope right or left. It should be noted that shifting the rear of the scope to the left will cause the point of impact to shift to the left and vise versa. It should also be noted that if one axis is near the limit of its adjustment, there will be a reduction in the amount of adjustment on the other axis. If the elevation adjustment is near the top of the adjustment range, the windage adjustment will be reduced; if windage adjustment has been induced, there will be a reduction in elevation adjustment. This can be illustrated by drawing a circle on a piece of paper to represent the maintube of a scope. If you start in the center of the circle with your pencil, you can move an equal distance in any of the four directions: up, down, left, or right. If you start in the center of the circle and move upwards toward the top of the circle, you will see that the distance remaining to the left and right has been diminished. The same is true in any direction; if you start in the center of the circle and move to the left, you will have diminished travel to adjust up or down.
** It is not uncommon for lower quality optics to have more adjustment travel than their higher quality counterparts for a number of reasons, but is typically due to the use of smaller, less expensive internal components. When smaller parts are placed in the same size housing (maintube), they will have the ability to move farther, but will also have negative aspects relating to image quality and durability.
