Leveling the Scope Reticle for Long Range Rifles
Leveling the Scope Reticle for Long Range RiflesBy Bruce Winker
©Copyright 2012 High Power Optics
Ask five different shooters how to level a rifle scope reticle and you’ll probably get five very different answers. Anyone who has installed a rifle scope has given this some thought and developed a process. Are all these methods just different ways of getting to the same result? Are there any special considerations for long range shooting?
This article looks at the unique requirements for leveling the reticle for long-range shooting. Different methods of leveling a reticle are compared and typical alignment errors are given for each method.
The long range rifle should be set up to eliminate or minimize all systematic aiming errors, such as canting errors. Canting errors occur when the rifle is zeroed at one elevation and then the elevation is changed to increase the range. Canting errors cause the bullet impact point to shift left or right as the elevation is increased above the rifle zero. There are two sources of canting errors, reticle alignment error and tilting the rifle during the shot.
Accuracy at long range requires that both of these systematic aiming errors be eliminated.
Through both experience and analysis the author has learned that not all reticle alignment methods give the same results. Some of the widely accepted reticle alignment methods can result in significant canting errors in long range shooting. To eliminate canting errors the vertical turret must be aligned so that vertical turret axis passes through the center of the rifle bore, as shown in Figure 1.
In the following discussion, we assume the scope reticle correctly aligned to the turret axes (i.e., no misalignment of the reticle during scope assembly), which is usually the case for all but the lowest quality scopes. Therefore, we could also say that the reticle must be aligned so that the vertical reticle axis passes through the center of the rifle bore. The issue of reticle misalignment inside the scope is discussed at the end of this article.
All rifle barrels have a measurable curvature of the rifle bore. The curvature may be small, but it’s there. In most rifles, and especially long range rifles with thick barrel profiles, the last few inches of the bore near the muzzle determine the point of impact. If possible, the reticle should be aligned to the rifle bore near the muzzle, not near the breech. While that is actually quite difficult to do, some alignment methods come closer than others.
If the reticle is not aligned to the rifle bore, then increasing reticle elevation will result in a small change in windage as well. The longer the target range, the larger the elevation angle needed to hit the target. Depending on the method used to align the reticle, the alignment error can be 4 degrees or more. The resulting windage aiming error can be 2 feet or more at long range.
In addition, canting of the rifle during the shot should be avoided by using an anti-cant indicator attached to the scope tube. The anti-cant indicator should be aligned so that it is level when the reticle is level. For long range shooting, this entire reticle alignment process (rifle/scope/anti-cant indictor) should be done with an error of less than +/-2 degrees. Recall that the big hand of the clock rotates 6 degrees for each minute, and you will realize how small this alignment tolerance is. There are several ways that reticle alignment errors can sneak in, and they can cause aiming errors of several feet in long range shots.
Figure 1. A: Proper reticle alignment to eliminate canting errors in long range shooting. Vertical reticle axis passes through the center of the rifle bore at the muzzle. B: Scope is offset to the left of the receiver. Even though the scope is aligned parallel to the receiver, the reticle is misaligned. C: The scope must be rotated to the left for proper reticle alignment.
One of the most common rifle misalignment problems is a horizontal offset between the scope tube axis and the rifle bore. This type of misalignment is common with most production rifles. It is caused by bent barrels, barrels mounted crooked in the receiver, and rail-mounted rings. Even a small curvature in the rifle bore can cause a significant offset at the receiver between the bore axis and the scope tube axis.
Weaver or Picatinny rings often cause horizontal offsets. These rail mounts are usually cut oversize to fit on non-standard rails. When the mount is tightened, the ring ends up shifting to the left or right of the rail. This happens with nearly all Weaver or Picatinny rings. One exception is Warne steel rings, which are split vertically and therefore mount with the scope tube always centered on the rail.
Figure 1 illustrates how a horizontal offset effects reticle alignment. Aligning the reticle to the rifle bore is the most accurate optical method (Figure 1.A), and eliminates canting errors due to reticle alignment. In Figure 1.B, the turret housing and receiver are parallel, but the scope is horizontally offset from the rifle bore, so the vertical reticle axis doesn’t pass through the bore. The scope must be rotated, as in Figure 1.C, to get proper alignment between the reticle and the rifle bore. The correctly aligned scope in Figure 1.C would appear misaligned to many shooters.
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