How to optically center a scope
By B.B. Pelletier
Today's post answers a question I received yesterday. To get the best performance from a scope, its adjustment knobs need to be in the optimum range, which is very close to the scope's optical center.
What is the optical center of a scope?
The optical center refers to the reticle and the field of view. An optically-centered scope shows zero reticle movement against a distant backdrop when the scope tube is rotated in a complete circle. Theoretically, that's possible to achieve, but I've never seen it. The best I've seen is a reticle that moves about a quarter inch against a target 20 yards away when the scope tube is rotated in a complete circle
Use a box instead of a gun to optically center your scope
I suppose you could rotate the scope in its rings if the rings' top caps (the top half of the rings) were removed and the turrets would clear the top of the gun when they came around, but I like another method.
With a small box sitting on a shooting bench, cut two v-shaped slots into the top edges and rest the scope on those two slots. Once the scope is focused on the target (which is 20 yards away), look through the scope and put a dot or mark on the target close to where the crosshairs are. It helps to have two people when you do this - one looking through the scope without moving it and the other downrange to draw the mark.
Rotate the scope tube around a complete circle and watch how the center of the crosshairs moves against your mark. It will probably move several inches at first. Figure out which adjustment knob to move and make your correction to reduce reticle movement against the target. This takes some time.
On the best day, I've adjusted a scope in 45 minutes. At worst, it took almost two hours to get it as good as it would get. Remember, it's almost impossible to remove the last bit of movement out of the reticle. Both the windage and elevation knobs will probably need some adjustment.
DO NOT think you can count the clicks of adjustment between lockup and complete spring relax and go to the center of that number for the optical center. It sounds good, but that way never works. Optical center is more precise than the center of the click adjustment range.
Once the scope is optically centered, put it in an adjustable mount like B-Square's AA adjustable mount and use the mount's adjustments to zero the scope and gun. Once you're zeroed, you'll still have all the scope's best range of adjustments remaining.
Is optical centering nesessary?
Absolutely not! You can just mount a scope and use it without going through this drill. But, if your scope runs out of adjustments or if you find yourself shooting to the right of the target at close range and to the left at long range, centering will cure the problem.