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Variable power scopes have been very popular with the shooting community for many years. These scopes should be mounted in a different way than fixed power telescopic sighting systems. The variable power feature places extra variables into the equation when shooting at distance.
Many have heard that you should sight-in a variable power scope on the magnification which will most likely be used when shooting. This is because most, if not all, variable power scopes will change point of impact when the magnification is changed. For the typical hunter that shoots game at less than 150 yards, the difference is usually inconsequential. For the varmint hunter or long range target shooter, the variation will make the difference between a hit or miss. As a result, for best accuracy at range, certain steps need to be taken.
First, the scope should be at the center of its adjustment range. Turn the windage adjustment knob all the way in either direction until it stops. Turn the knob back until it contacts the stop on the other end of its adjustment range, counting the revolutions as you go. Suppose the complete range is 20 revolutions. In this case, turn the adjustment knob back 10 revolutions and you are at the center of the adjustment range. Do the same with both adjustment knobs.
The adjustments for zeroing the scope should be done primarily with the mount system. Several companies offer adjustable mounts. Some offer adjustable rings. The most common mount is the rotary dovetail type in which the front ring is inserted into the front base and rotated 90 degrees to lock into the base. The adjustable rear base has a screw on either side of the base to allow the rear ring to swing right or left and tighten the ring in place. This eliminates any need to adjust windage with the internal scope adjustments. Elevation is not as critical, but can also be adjusted by placing shims under the front or rear of the base or bases. The Weaver or picatinny style bases are fixed, so adjustable rings are required. These adjustable rings are somewhat difficult to find, but they are available.
Adjustable scope mount systems are more expensive, but to get full advantage of a variable power scope, they are a necessity when shooting long range.