South Texas Marksmanship Training Center
SELECTING A RIFLE SCOPE
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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 main-tube 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 (main-tube), they will have the ability to move farther, but will also have negative
aspects relating to image quality and durability
.

In the diagrams below, the outer circle represents the main-tube of the scope when looking through the optic. The black
dot represents the erector system, or internal lens cluster that is moved when making windage/elevation adjustments on
the scope. The thin lines help illustrate the amount of travel remaining on the opposite axis as windage/elevation
adjustments are made; helping illustrate how an adjustment in one direction limits the amount of travel in the other.

























·        1. Represents a scope with the erector system in the center of the adjustment travel range; allowing
for maximum adjustment travel on the elevation axis (the same is true for windage when the erector system
is centered).
·        2. Represents a scope with the erector system in the center of the adjustment travel range; allowing
for maximum adjustment travel on the windage axis (the same is true for elevation when the erector system
is centered).
·        3. Represents a scope with the erector system near the end of the available travel in the up direction;
drastically reducing the amount of windage travel remaining.
·        4. Represents a scope with the erector system near the end of the available travel in the left direction;
drastically reducing the amount of elevation travel remaining.
·        5. Represents the scope on rifle #1 in the above example; when sighted-in at 100 yards.
·        6. Represents the scope on rifle #2 in the above example; when sighted-in at 100 yards.
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