Fig 1. Collimator and spud inserted into rifle bore
Select the proper caliber spud. Insert the spud shank into the Site-A-Line vee, with the bronze spring on the opposite end, pointing upward.
Next, insert the spring-end of the spud into the gun's muzzle. tThe spring will hold the spud securely to the bottom of the bore - on top of the rifling lands - helping to prevent the instrument and spud from rotating below the line of slight. It is not necessary to make the Site-A-Line plumb with the firearm in rotation. Regardless of the instrument's position, it is always parallel to the line of sight as it will only use parallel light.
|Fig. 2 - aligning the diagonal
with scope crosshairs
When the sights are adjusted to coincide with the center of the collimator reticle in the optical field of the Site-A-Line, the last two and a half inches of the gun bore (which controls the flight of the bullet) is parallel with the line of sight; optical and mechanical zero has been achieved.
range firing may show a small amount of error between the shot group center and target aiming point. Often only a half-inch correction is necessary at 100 yards. Load variations or other factors may require a slight additional adjustment to hit center.
For example, at 100 yards, if an error of minus 2" drop and 1" windage is noted, it is a simple matter to jot down the corretions for this particular rifle and load and if the sights are subsequently disturbed they need only be raised two minutes of angle and adjusted one minute left windage from Site-A-Line zero to center the ground dead-on at 100 yards.
To adjust for 200 yards, the windage will remain the same. Elevation will be raised approximately 2 1/2" for a .30-'06 trajectory. If scope or iron sight is graduated in 1/4 minute clicks the sight brouhgt up 10 clicks from the corrected 100 yard zero. Little or no correction is necesary for caliber and rifle combinations with very flat trajectories.
Please also see our FAQ for additional information.