Wednesday, January 4, 2023

Prepper's Armory: Scope Zeroing

Now that the scope has been properly mounted to the firearm, it's time to adjust the optic so that Point of Aim (POA) matches Point of Impact (POI) for a specific distance. This is called zeroing the scope, and while this can be approximated on the work bench, the only way to truly zero a scope is from the shooting bench.

Minute of Angle
Scope knobs apply adjustments in ¼ or ½ minute of angle increments. A Minute of Angle (MOA) is defined as 1/60th of a degree, which equates to approximately 1 inch per 100 yards (actually 1.047″) and increases with distance. Therefore, 8 inches at 800 yards is still 1 Minute of Angle.

There are two main categories of bore sight tools available on the market today: analog and laser. The analog version is a collimator type system held in place either by a magnet or barrel spud. In either case, a grid is displayed in the field of view of the scope.

Spud and Magnetic Collimator Boresights

There are two types of laser boresight. The arbor version is similar to the analog collimator in that a spud is inserted at the muzzle end of the barrel using a caliber-specific bushing. After it's in place, the laser is activated and projected on a surface a set distance away.

The chamber type is shaped like an empty shell casing and inserted into the chamber. Once activated, it's treated in the same manner as the other laser type mentioned above.

Arbor and Chamber Laser Boresights

Zeroing a scope using either method is easier if the rifle is solidly secured, either through a vice, sandbags, or some other fixture.

When sighting in a rifle, the specific range chosen will affect the functional zero -- the two different distances where the bullet will hit at approximately the same point on the target, due to its parabolic rise and fall -- without having to adjust elevation or holdover excessively. (Holdover refers to raising the crosshair when aiming at a target further away than the range for which the scope is zeroed. This is one of the uses for Mil-Dots on a reticle.) The image below provides a visual for this concept.

Zero ranges for the 5.56mm cartridge from Gun Websites

Analog Boresight
Once installed and adjusted according to manufacturer instructions,  carefully rotate the windage and elevation knobs on the scope so the crosshairs line up with the center of the collimator target. Some analog boresights can be adjusted to estimate a specific range, but most are fixed.

Image courtesy of Savage Arms

Laser Boresight
Regardless of whether you are using arbor or chamber type, a laser boresight projects a dot onto a surface at a set distance. As with the above method, carefully rotate the windage and elevation knobs on the scope so that the crosshairs line up with the dot. This method can result in smaller groups when shot testing, as the sighting point is not right in front of the firearm.

Old-School Boresighting Technique
Last, but certainly not least, is the literal boresight method. While this technique isn't as precise as either of the two above, it requires no tools other than a method of securing the firearm.
  1. The bolt is removed to allow an unobstructed view through the barrel from the chamber end. (This may not be possible on all rifles.)
  2. The bore is centered on an aiming point at a set distance. 
  3. Without disturbing the rifle, the windage and elevation knobs on the scope are carefully rotated so the crosshairs line up with the center of the selected target.

While all these techniques can put shots on paper, as mentioned above the only way to truly zero a firearm is by actual shooting. However, beginning the sight-in process with bore sighting can save time and ammunition -- two things that are often in short supply.

Good luck, and safe shooting.

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