Laser Bore Sighters for Firearms: Only Suitable for Short Distances

A laser that is inserted into the barrel of a firearm is a useful tool for the coarse pre-adjustment of the sighting device. However, this method is unsuitable for precise adjustment over long distances. The reasons for this lie in the fundamental differences between the trajectory of a projectile and the path of a laser beam, as well as in the physical properties of the laser itself.

Straight Laser Meets Curved Trajectory

The most critical factor limiting the usefulness of a bore sighter to short distances is the ballistic curve of a projectile.

• The Laser Beam: A laser beam travels in an almost perfectly straight line. Aside from minimal scattering and atmospheric effects, which are negligible at typical shooting distances, its path can be considered a straight line.

• The Bullet Trajectory: From the moment it leaves the barrel, a fired projectile is subject to gravity. As a result, it follows a curved, ballistic trajectory. The bullet initially rises slightly (relative to the line of sight), reaches an apex, and then falls in an increasingly steep arc.

A bore sighter merely simulates the exact extension of the barrel's axis. At any other distance, the bullet will impact either above or below the laser dot due to its curved trajectory.

Image Caption: This schematic diagram shows the difference between the straight path of the laser beam and the curved, ballistic trajectory of the bullet. While the laser draws a straight line to the target, the bullet drops due to gravity.

Physical Limits of the Laser

In addition to the ballistic discrepancy, other technical limitations restrict its use over long distances.

• Beam Divergence: No laser beam is perfectly collimated. With increasing distance, the light beam spreads out, and the dot becomes larger and less focused. At a distance of 100 meters or more, the laser dot can become so large and diffuse that a precise alignment of a scope's fine reticle becomes impossible.

Image Caption: This image illustrates beam divergence. The laser dot becomes larger and more blurry with increasing distance, which makes precise aiming at long ranges difficult.

• Visibility: The intensity of the laser dot decreases with distance. Especially in daylight, the small red or green dot is difficult or impossible to see at long ranges. This makes a reliable adjustment under real-world conditions practically impossible.

Parallax and Mechanical Tolerances

Other factors that influence accuracy are:

• Parallax: With scopes, a parallax error can occur. This means that the reticle appears to move on the target if the shooter does not position their eye exactly centered behind the eyepiece. Although this is an issue with the scope and not the laser, it must be corrected when zeroing at the target distance, which is not possible with a laser adjustment alone.

Image Caption: The graphic explains parallax error: If you do not look exactly through the center of the scope, the reticle appears to shift on the target. This leads to aiming errors.

• Fit and Tolerances: The arbors and adapters used to center the laser in the barrel always have some degree of mechanical play. These minimal tolerances may be negligible at short distances, but they lead to a significant deviation of the laser dot from the actual barrel axis at longer ranges.

Conclusion: A Tool for Coarse Adjustment

A laser bore sighter is an excellent tool for coarsely pre-adjusting a firearm. It ensures that the first shots at a short distance (typically 25 meters) will already be on the target paper. This saves time and ammunition during the subsequent, essential process of sighting in with live ammunition at the desired zeroing distance. However, it can never replace actual shooting, as only live firing takes all real ballistic influences into account.