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The following formula is useful for calculating what thickness shim you need to use to gain a specific minute of angle adjustment when mounting a scope.

Take the desired MOA adjustment and multiply it by the base length in inches. Divide your answer by 3600 and that will give you the thickness of shim to use. Shim under the rear of the base to move your point of impact up and shim under the front to move your POI down.

How you measure your base length is critical. If you are shimming a one piece base at the rear, measure from the forward most point where the base contacts the receiver to the forward most part of the shim. If you are shimming the front, measure from the rearmost point where the base contacts the receiver to the rearmost point of the shim. Because the rings will remain in alignment it is unnecessary to lap them again after shimming. You may wish to glass bed between the base and the receiver to provide more contact otherwise the base will only be supported at two narrow points.

When using a two piece base or shimming the scope rings, measure between the centers of the rings. To keep from stressing the scope you will want to lap the rings after shimming a two piece base. You could also use Burris Signature rings with the alignment inserts or glass bed the scope to the rings.

Ideally the base and rings should be machined in perfect alignment and contact with the rifle and scope to begin with. Aftermarket bases with a built in down-angle for thousand yard shooting are available.

The formula also works for adjusting iron sights. Use the sight radius for the base length. Move the rear sight in the direction you want your POI to move. Move the front sight in the direction opposite the direction you want your POI to move.
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