Shim Thickness Changes by X MOAs

The importance of preserving elevation MOAs of adjustment becomes clear with a specific example. Let's say your scope has 30 MOAs total elevation and you have a .308 rifle with a 300-yard zero. After zeroing, you find you have 10 MOAs down and 20 MOAs up, but in order to engage targets at 100 yards you need -5.25 MOAs, and to fire at 800 yards you require +23 MOAs. You need three more up MOAs to adjust for 800-yard shots.

Therefore, by inserting a .005-inch shim under the mount's front, you will have in effect "shifted" 3.5 MOAs and can now adjust elevation almost perfectly from 100 through 800 yards. That's quite a result for a paper-thin piece of brass!

You won't know for sure what the final up/down MOAs will be until after you've zeroed, and this means you may have to completely remount the base in order to insert a shim, then boresight again and rezero. It's rime-consuming but worthwhile.

On the other hand, this should verify the importance of having a scope with plenty of up/down elevation MOAs for your round's trajectory so you have enough margin to make adjustments and don't even need shims.

Another option is offered by Burris, the Pos-Align Offset Insert Kit, which contains shimlike inserts that fit inside the company's Signature scope rings. These scope ring inserts precisely shift elevation or windage, depending on how they're installed.


Each time a scope is remounted on a rifle, then fine-adjusted for an exact zero, an inevitable by-product is that a few clicks of windage or elevation have been shifted from the scope's mechanical center.

After several remount-ings, a significant amount of internal adjustments— especially elevation—may have been consumed, reducing the potential outer adjustment limits and therefore the maximum distance at which you can click in to engage a target dead-on.

Restoring a reticle to its factory-set mechanical center is very simple if there are an equal number of up/down or right/left MOAs. All you need do is turn the adjustment all the way to one end, then come back in the reverse direction and count the number of clicks. Now that you know the total clicks, divide by half and turn the knob back that far.

If you have an unequal number of MOAs—such as 45 up and 15 down elevation—it's a bit more complicated but no problem using the following technique.

Cut two Vs of similar height in a narrow cardboard box so it can hold your scope, as shown in the top illustration on page 113. After removing the elevation and windage turret covers, place the box and scope on a flat surface like a bench and tape it down.

Now, aim the crosshairs at a precise spot 50 yards or farther away. Starting with the elevation turret up, and being very careful not to move the box, rotate the scope while watching the crosshairs.

You will see that the crosshairs will trace an elongated circle or elliptical path as you com-

Elevation and up/ down compensation use the vertical mil scale.

Moa Scope Rings Shim


yards, 1 inch equates 10 1 MOA. No conversion's required. Since your scope has 1/4 MOA adjustments, that means you must come up 12 clicks (3 inches = 12 clicks @ 1/4 MOA). That was simple.

Now, though, you're firing at a target 500 yards away, and your spotter reports your round hit 6 inches high. You stroke your chin. Let's see, an MOA at 300 yards equals 3 inches, at 400 yards it would be 4 inches, and at 500 yards 1 MOA equals 5 inches. So, to lower the point of impact by 6 inches at 500 yards, I'd have to go down 1 1/5 MOAs. My scope has 1/4 MOA clicks, and therefore I go down 5 clicks to be as close as possible (5 clicks = 6.25 inches).

Next let's compute a windage change. Your bullet has impacted 4 inches left at 700 yards. How many clicks in what direction do you adjust? (Answer below.)

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Hunting Mastery Selected Tips

Hunting Mastery Selected Tips

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