Reticle Alignment Canting Errors. The author has installed scopes on a wide variety of rifles, and found that production and semi-custom rifles are prone to a host of alignment problems, usually due to high volume manufacturing and assembly of receivers and barrels. Each of these rifle alignment flaws can cause the reticle not to point through the center of the rifle bore, which leads to a canting error. Some of the alignment methods are more sensitive to these alignment errors than others, as discussed above. The boresight method (2) is insensitive to most of the rifle alignment flaws, for example, while the last method (5) is sensitive to almost all of them.
Alignment errors result from several common manufacturing and scope installation “flaws”:
• Bent barrel (2-10 MOA misalignment is common in production rifles).
• Barrel not true to receiver (4-12 MOA misalignment is common in production rifles).
• Point of impact offset from bore direction (resulting from thin barrel profile or poor stock fit).
• Scope base not true to receiver (up to 6-10 MOA misalignment is common due to screw hole size and location tolerances).
• Rings not parallel to scope base (common with windage adjustable dovetail rings).
• Rings not centered on rail (up to 0.040” offset is common with Weaver, Picatinny, etc.).
Only rifles that have been carefully boresight aligned by a qualified gunsmith using a trued action, custom barrel, bedded stock, etc., are free from most of these alignment flaws.
Table 1 lists typical reticle alignment and aiming errors for each of the five reticle alignment methods. By “typical” we mean that the total reticle alignment error was calculated assuming typical values for each type of misalignment that applies to each reticle alignment method. Any given rifle could have all or none of these problems, however, and the magnitude of the misalignments could more or less than the “typical” values used. Also, these misalignments can sometimes produce errors in opposite directions and cancel each other out to some extent. Therefore, the horizontal aiming error for any given rifle could be more or less than the values listed in Table 1. The aiming errors in Table 1 should be used for comparison purposes, and not as an indication of absolute accuracy for a given reticle alignment method or rifle.
Table 1. Typical reticle alignment and horizontal
aiming errors for short and long range shots. Ballistics for 7 mm WSM at sea level were used in these calculations. Many long range cartridges have generally similar ballistics.
Table 1 shows how horizontal aiming errors increase with range. This is because of the compound effect of increasing elevation adjustment and increasing range. The Table shows that most of these alignment methods are adequate for short range shots. However, at 700-1,000 yds the aiming error can be 0.5-2 feet if an inaccurate reticle alignment method is used.
These canting errors can either add or subtract to canting errors due to tilting the rifle during the shot. For example, a rifle with a 4 degree reticle alignment error, fired with a 6 degree rifle cant in the same direction, would have an aiming error of 21 inches at 700 yds, and 54 inches at 1,000 yds! Canting errors of 4-6 degrees are almost imperceptible without an accurate alignment reference.
Boresight Alignment Method in Detail. The first step, horizontal boresight alignment, involves adjusting the scope base and ring alignment so that the scope tube axis is parallel to the rifle bore at the muzzle. In high quality production, semi-custom and custom rifles, the point of impact is usually within about 4 MOA of the bore axis at the muzzle. Lower quality, high volume production rifles can have bent barrels, excessive barrel resonance and poor stock fit that can cause the point of impact to fall more than 10 MOA away from the bore axis. We assume the long range shooter is using a rifle with acceptable quality in materials, machining and assembly. If not, boresight alignment should be confirmed relative to the point of impact, rather than the rifle bore at the muzzle.
Gunsmiths use a variety of methods to insure that the rifle has no significant boresight misalignment problems. Mechanical methods include mounting the barrel and receiver on a mill table and using a dial indicator to test trueness of the barrel and action, and the use of Kokopelli alignment bars. Optical methods include the use of a quality boresight collimator (such as the Bushnell Professional Boresighter or the Sweany Site-A-Line Boresighter) to determine the bore axis, and then aligning the scope tube axis parallel to the collimator axis. Once this process has been done, it does not have to be repeated, as long as care is used when changing scopes. The long range shooter can perform the horizontal boresight alignment process using commonly available measurement tools and either Kokopelli alignment bars or a calibrated boresight collimator. A reticle crosshair that is aligned to the point of impact at short range and still has a large windage offset (>8 MOA) from the mechanical center is evidence of boresight misalignment.
Figure 4. Left: The RingTrue™ Alignment Tool properly aligned and leveled. Double stick tape holds the Alignment Tool against the objective rim. Right: The inverted alignment tool can be used to level the elevation turret.
The reticle alignment process has two parts. First, the rifle is rotated until the scope tube axis is positioned vertically above the rifle bore axis. If there is a boresight misalignment between the rifle bore and scope tube axis, for example, the reticle should be aligned as close to the muzzle as possible. Therefore it is better to use the objective end of the scope, rather than the eyepiece end, for this alignment process.
This step is easily done using either the EXD or RingTrue Alignment Tool (see Figure 4). The alignment tool is aligned to the barrel and objective bell. The rifle is rotated until the spirit level on the alignment tool is level, and then the rifle is secured in this position.
Second, a spirit level is placed on the turret or turret cap, and the scope is rotated until the turret assembly is level. At this point the rings cap screws are tightened using a torque wrench, while checking that the scope does not rotate. Small reticle alignment errors may result from residual boresight misalignment errors, bent barrels, barrel resonances that cause a point of impact shift, as well as from errors in the spirit levels. The resulting reticle misalignment is typically no more than 1.0 degree if the rings are boresight aligned to the rifle bore.
In some scopes there is a small alignment error between the reticle axes and the turret axes. If this is the case, the final reticle alignment process varies depending on the type of elevation adjustment that will be used. The boresight reticle alignment process we just described forces the turret axis to pass through the center of the bore. This method is preferred when the turrets are dialed for elevation. If instead a bullet drop compensated (BDC) reticle is used for elevation, align the reticle to a plumb line or level line located at least 20 ft in front of the rifle. A boresight collimator with an attached spirit level is very handy for this purpose.
Finally, an anti-cant indicator should be attached to the scope tube and aligned. Canting errors due to tilting the rifle during the shot are usually larger than the canting errors due to reticle misalignment. Anti-cant indicators that attach to the Weaver or Picatinny rail should not be used. It should be clear by now that the top of the rail is not necessarily parallel to the reticle when the reticle is properly aligned.
Bruce Winker lives on the West Coast where he is an inventor and scientist with over 20 years experience developing advanced optical displays, weapon sights and fire control systems. He has a Ph.D. in Analytical Chemistry. At work he specializes in optics and ballistics. Bruce is also Vice-President of High Power Optics. He is committed to helping shooters in all disciplines get the best performance from their optics. He runs free rifle scope clinics for shooters every month at local shooting ranges. He is an avid outdoorsman who enjoys fishing and hunting for deer, wild big, varmints and predators. Bruce is a husband and father of two kids.
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