It's frustrating when you have a target perfectly aligned in your rifle's crosshairs, but when you pull the trigger the bullet ends up several inches or even yards away. While this could be attributed to wind, a loose grip, or having a dirty barrel, it's often caused by a misalignment of the optical sights. If the rifle's optical or iron sights aren't aligned with the barrel, the bullet isn't going to land on target.
How Boresighting Works
You can align the barrel of your rifle to the sights, through a process known as boresighting. The traditional method of boresighting lives up to its namesake in the sense that you literally look down the bore of your rifle. The rifle is secured to a fixed station, at which point the bolt is removed so the shooter can see down and through the barrel. Adjust the rifle so you can see the target dead-center through the barrel, and then do the same for the sights. There's no need to actually fire the rifle when using the traditional boresighting technique, as it only involves looking through the barrel and sights until they are both on target.
It's paramount that the rifle is kept stationary during this process, as even the slightest movement can throw off the alignment. Some shooters prefer to use benches or other stabilizers to keep their rifle still when boresighting.
Of course, there are several newer boresighting techniques as well, one of which involves the use of a collimator and arbors. The collimator is an accessory with a small sheet of paper with horizontal and vertical grids that is placed inside the barrel of a rifle. The arbors hold the collimator in place while you shoot. To boresight using this method, the shooter looks through his or her sights, adjusting the windage and elevation until the target is dead-center. The collimators are used to recheck the zero after shooting.
Among the newest, and arguably easiest, boresighting techniques is laser. Laser boresighting involves the use of a laser to illuminate the target, eliminating the need to manually remove the bolt. Some laser boresights are inserted into the rifle's barrel and held in place with arbors, whereas others are inserted into the barrel like a standard cartridge (requires precise size according to your rifle's caliber). Many shooters prefer laser boresighting because of its ease of use.
Whether you choose the traditional method, collimator, or laser, boresighting remains a time-tested and effective way to align your rifle's sights with the barrel. With that said, it's important to remember that certain ammo types will shoot differently than others. Even if you boresight your rifle using a technique mentioned here, you should still test it at the shooting range to ensure the sights and barrel are properly aligned.