Now we summarize the various methods used for reticle alignment. Commonly used reticle alignment methods can be divided into five basic groups.
Figure 2. Live fire reticle alignment method. A: The reticle is rotated until the point of impact falls on a plumb (vertical) line above the zero. B: The reticle is misaligned, producing a canting error at 100 yds. The reticle needs to be rotated counterclockwise.
1. Live fire method. This method takes a lot of time and a fair amount of ammunition, but is the most accurate reticle alignment method. First the rifle is leveled and zeroed. Then a target with a long plumb line (at least 16 inches, but preferably 32 inches) is set up at a fixed range, say 100 yds. A group of shots is fired while aiming at the bottom of the plumb line with the scope elevation set to zero.
Then the elevation is increased by 15-30 MOA and another group is fired while aiming at the same target position (bottom of the plumb line). It is important that reticle be level while firing this group. The second group should fall on the plumb line, 15-30 MOA directly above the first group (see Figure 2). If the second group falls to the left or right of the plumb line, the scope must be rotated clockwise or counterclockwise, respectively. This process of firing a group and adjusting the scope is repeated until the group falls directly on the plumb line passing through the rifle zero point of impact.
This method results in almost perfect reticle alignment. The only errors are due to inaccuracy of the spirit levels used to level the rifle while shooting, in determining the center of the group. The potentially large amount of ammunition, time and discipline required to perform this process properly warrants using an optical method.
2. Boresight method. This method aligns the reticle to the bore of the rifle and is the most accurate optical method. First, it involves horizontal boresight alignment of the scope tube axis to the rifle bore axis, followed by alignment of the reticle to the rifle bore. This method is difficult to do without an alignment tool, however. Two alignment tools are now available that allow accurate alignment of the reticle to the rifle bore. The EXD Alignment Tool is sold by Brownells ($46), and the RingTrue Alignment Tool is sold by High Power Optics ($26). Both tools include accurate spirit levels that allow alignment of an anti-cant indicator after the reticle is aligned. The boresight alignment method is described in more detail below.
This reticle alignment method is very accurate (less than +/-2 degree of error). Small reticle alignment errors can result from a bent barrel or a barrel being mounted crooked in the receiver, which causes a small offset of scope axis from the bore at the muzzle. Compared to other methods, however, this method is less sensitive to these rifle alignment problems.
3. Reticle alignment to rifle bore. This method is the same as the method 2 above, except that the horizontal boresight alignment step is omitted. Like the full boresight method, this method requires either the EXD or RingTrue Alignment Tool.
Accuracy is good for all but the longest shots (greater than about 800 yds). Reticle alignment errors do not exceed +/-3 degrees. Figure 3 illustrates why boresight alignment is so important. Even though the objective end of the scope is aligned to the rifle bore, there may still be a reticle alignment error due to the scope tube axis not being horizontally parallel to the rifle bore. When this occurs the objective can be offset from the bore, even though the elevation turret is centered over the receiver. The offset of the objective causes a small reticle misalignment error. Correcting the boresight misalignment eliminates this source of error in the reticle alignment process.
Figure 3. Effect of horizontal boresight mis-alignment on reticle alignment (exaggerated for illustration). In this illustration, the turret housing is centered over the rifle bore and the reticle is properly aligned. The angular misalignment causes a horizontal offset between the ends of the scope and the rifle bore. This apparent offset results in a reticle alignment error using Methods 2-4. Correcting the boresight misalignment eliminates this source of error in the reticle alignment process.
4. Reticle alignment to the rear of the receiver. This method is easy to do by looking into the eyepiece and rotating the scope until the reticle appears to point down toward the center of the receiver. B-Square sells the Cross Hair Reticle Alignment Tool ($10) which is a flat strip of clear acrylic sheet that is bent at a right angle. One end slips into the rear receiver of a bolt action rifle, while the other end has a cross hair printed on it. The user looks through the acrylic sheet and aligns the cross hair parallel to the reticle in the scope. This device only works with bolt actions and does not fit a wide variety of rifles and scopes well. Parabola sells the Reticle-Tru alignment tool ($65) which attaches to the eyepiece. It has a large arrow that is rotated to point toward the center of the receiver, and a thin slot that allows alignment of the reticle to the receiver.
Accuracy is adequate for short-medium range, but can produce significant canting errors for long range shots. Because the alignment is done at the eyepiece end of the scope, rather than the objective end, this method is more sensitive to boresight misalignment errors than the previous methods. It is also more sensitive to the barrel being mounted crooked in the receiver, which is a common problem in production and semi-custom rifles (whenever the action is not trued). A spirit level or plumb line is still required to level the reticle when aligning an anti-cant indicator.
5. Scope alignment to the top of receiver. This is probably the most common optical alignment method. Several variations exist and at least one of them is familiar to most shooters. This method usually involves the use of spirit levels of one type or another, some of which can be quite expensive. A level is placed on the scope base (or rail), or some other flat surface on the top of the receiver, and the rifle is rotated until the receiver is level. Then a spirit level is placed on the rifle scope turret cap and the scope is rotated until the turret cap is level.
After leveling the receiver or base, some shooters use a plumb line 20 or more feet in front of the rifle to align the scope reticle. Another variation is to use a metal ruler to align the bottom of the turret housing parallel to the scope base (or rail). The Segway Reticle Leveler, sold by Straight Shot ($20), is a handy tool that has bold horizontal black lines and attaches to the receiver (usually the base or rail) using a rubber band. It enables the alignment of the reticle by looking into the eyepiece and the leveler at the same time.
All of the variations of Method 5 do essentially the same thing and produce equivalent results. Accuracy is adequate for short range, but this type of method can have large canting errors that make it a poor choice for long range shooting. This method is sensitive to many boresight alignment problems. Canting errors occur whenever the scope tube axis offset from the rifle bore, as shown in Figure 1.B.