With any rifle, it is important to have an understanding of your setup and equipment. For precision rifle, when shooting inside your zero range or when you are shooting from a port or behind an obstacle, understanding what mechanical offset is and why it is important can make the difference between you effectively taking a good shot or whether you shoot or damage the port or obstacle you were behind!
So, What is “Mechanical Offset”?
Mechanical offset is the difference between your optics and the center bore of your barrel. When required to clear an obstacle, for example when using a shooting bag for a rest, the height of the scope over the barrel becomes important. If you have a clear sight picture in the scope, but then your bullet impacts the obstacle in front of you, you failed to account for mechanical offset. You can verify your clearance by initially sighting down your bore when getting into a particular position.
Inside 100 yards, the mechanical offset determines the hold for your shots. When hunting, this can be an issue with rocks, trees, or other foliage that are inside your shooting position. From behind any barricade or using a shooting bag as a rest, the bullet that passes through the obstacle will not have an accurate placement if it needs to penetrate through something first.
The mechanical offset can vary with each firearm, but for rifles it is usually between 1” and 3”. Not all rifle sights will have the same distance from the bore axis. They can vary in accordance with any risers or scope rings and mounts used. Rifles can exhibit differences in points of aim and point of impact at closer distances, especially carbines where the front sight is usually 2.5 inches from the bore. If you are looking for the exact matches for your needs, a custom rifle is usually the best bet.
For example, when zeroing your rifle, your point of aim could be dead center on a circle target. Yet, you may notice that the point of impact is below your line of sight due to the descending line created by your optic. So if you want to hit your target at close range, you need to compensate for mechanical offset based on the distance from which you are shooting.
Original Source: https://gunware.net/precision-accuracy/mechanical-offset-important/