Boresight vs. Zero

At HighPowerOptics we use the term “zeroing” to mean adjusting the point of aim (POA) to coincide with the point of impact (POI). Zeroing an optical or iron sight is relatively easy to do. Tools are available to help the shooter get the POA close to the POI without firing the rifle. This process is often called “getting on paper”. Accurate zeroing always requires live fire.

Boresighting a scope to a rifle, however, is very difficult to do without special tools. We use the term “boresighting” to mean aligning the optical axis of the rifle scope to the axis of the rifle bore (see Figure 4).The optical axis of the scope is also the same as the rotational axis of the scope tube. In practice, boresighting also includes aligning the rings to each other. Our RingTrue™ scope installation process accom

Figure 4. Boresighting a scope to a rifle. Dotted lines represent the axis of the bore and the optical axis of the scope.

Why boresight a scope to a rifle? All rifle scopes have a limited useful internal reticle adjustment range (see Figure 5). At about 0.5 degrees (30 MOA) of reticle adjustment from the optical center, the image starts to blur due to optical aberrations, and image contrast starts to degrade due to an effect called veiling glare. Boresighting the scope keeps the reticle closer to the optical center, preventing unnecessary loss of resolution and contrast.

Also, canting errors result when the reticle is not correctly aligned to the rifle bore – even if an anti-canting level is attached to the scope. To prevent these canting errors, the projection of the reticle must go through center of the rifle bore. The optical process for aligning the reticle this way requires the scope to be boresighted horizontally. Otherwise, the reticle must be aligned using a time-consuming and costly live fire method.

Figure 5. Reticle adjustment range.

Why is boresighting necessary? Machining errors in modern production rifles and bases can result in a misalignment between the rifle bore and the scope attachment point(usually a base attached by machine screws to the receiver) of more than 0.25 degrees (15 MOA). Without boresighting the scope to the bore, much of the useful internal adjustment range can be used up correcting for this misalignment (see Figure 6). This problem can limit the range of your rifle by reducing the effective adjustment range of the reticle.

Figure 6. Poor boresight alignment between scope and rifle.
Left: Rifle bore axis (point-of-impact) is far from the scope optical axis.
Right: Half of the vertical adjustment range is needed to position the reticle over the point of impact.

A properly boresighted rifle has the optical axis of the scope aligned to the bore of the rifle so that all of the reticle adjustment can be used for bullet drop and windage adjustments. Boresighting enables you to get the optimum optical performance from the rifle scopeand eliminates canting errors due to reticle misalignment, especially when shooting at long ranges. Put another way, it enables the shooter to achieve the longest possible target range utilizing the scope’s reticle adjustment range.

The RingTrue™ scope installation process enables you to determine if the base is adequately aligned to the bore. If not, we provide guidance on correcting the misalignment. Options include shimming or bedding the base, changing bases, or using adjustable rings, such as Burris Signature™ rings.