Zeroing Discussion Part Two: Why we Boresight at 10 and Zero at 25
For Walkthrough Wednesday, we are continuing our discussion on how to zero the M4A1 Carbine and coach our Paratroopers through the zero process. Specifically, we will be addressing how to boresight. The reference for this discussion is appendix E, TC 3-22.9, Change 2, dated August 2017.
As a refresher, the borelight is an eye-safe laser that is used to boresight optics, iron sights, and aiming lasers. The borelight assists the first shot group hitting the 25-m zeroing target when zeroing the weapon. Utilizing the borelight will save range time and require fewer rounds for the zeroing process. Borelighting is done with a borelight, which is centered in the bore of the weapon, and with an offset target placed 10 meters from the muzzle of the weapon.
Before we can Boresight our weapon, it is essential to grasp the concept of the interactions that take place to hit a target. An effective way to illustrate this is by discussing each one of these terms in order, and how they affect ballistics, starting with the Line of Sight.
To put it simply, Line of Sight (or Gun Target Line) is the straight line from the eye through the aiming device (Scope, Iron Sights, Laser) to the Point of Aim. There is another line we are concerned with, and it is the Line of Bore, also known as the Line of Elevation. It is the line defined by the bore of the rifle or the path the bullet would take without gravity. In other words, if you were to take the weapon apart and be able to look through the barrel while it is sighted on the target, this would be what you would see.
Why the Line of sight and the Line of Elevation are essential are because of the factors that affect the round once it exits the barrel. When a projectile exits the muzzle of a rifle, it drops from the line of elevation, otherwise known as the Axis of Bore
As the projectile travels down range, the velocity is decreased by air drag, thus giving way to the inevitable force of gravity. It is this effect, which creates trajectory, otherwise known as a parabolic curve.
So essentially, when we zero our weapon, we are adjusting our line of elevation, so that it creates a trajectory whose Point Of Impact (POI) intersects with our line of sight at the Point Of Aim (POA) on the target. There are certain crossover points in the round’s trajectory, where POA is very close to POI. They are at 10 and 25 meters, hence the reason we Battlesight zero and Borelight at these distances.
So to sum up, we’ve discussed why we zero and boresight at ten and twenty-five meters. Leaders of all levels need to educate themselves now on this so that they can be more efficient in their use of one of our most valuable assets: range time. Next week, we will continue our discussion on zeroing as we continue to discuss how to use the boresight and offsets.
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