The AR-15 platform is so popular these days; it is often referred to as “America’s Rifle” or the “Modern Sporting Rifle”. Its popularity is easy to understand as it is light weight, accurate, low recoil, and incredibly modular. There are a few aspects of the rifle that need to be understood when it is employed at very close ranges, shooting around barricades, and shooting from non-standard positions. One of these aspects is the mechanical offset between the rifle’s bore, and the rifle’s sights.
The photo here shows what mechanical is on your rifle and the article describes why it's an important concept to know.
What is “mechanical offset”?
Mechanical offset is the height difference between the bore of the rifle, and the sights on the rifle. The difference is approximately 2.5” (2.6"-2.7"), and can lead to problems in where the rifle is aimed (POA or point of aim), and where the round will impact the target (POI or point of impact).
Traditionally the rifle’s sights (or optic) are sighted in while the rifle is vertical, that is, the sights are in plane, directly above the bore. The POA and POI are matched at a determined distance where the bullet’s parabolic flight path impacts the target at a point where the sighting system is lined up. Because the bullet is affected by gravity, it does not fly in a true straight line, but rather arcs from where it leaves the barrel to where it finally reaches the deck.
This illustration shows the flight of a bullet in relationship to the point of aim. Because of the effect of gravity, the trajectory of a bullet is always going to be an arc.
Because the AR platform has a rather large mechanical offset, POA and POI can be significantly affected when the rifle is used in close distances. Note the POA vs. Trajectory graphic. This illustrates the differences between where the sighting system is aligned and where the round will impact while the rifle is aimed traditionally. When used in distances closer than the original zero distance, the rifle will need to be aimed higher to achieve a POI in the desired location. At a distance of zero the round will be impact low equal to the amount of offset between the muzzle and the sighting system. This distance will continue to decrease until the target is at your zero distance.
One of the biggest reasons to keep mechanical offset in mind is when shooting around barricades.
Most times when you are using a barricade, you are trying to minimize the amount of your body that is visible to the target. This leads to just barely seeing over/around/under a barricade to get your sight alignment. Without being aware of the mechanical offset of your sighting system, this can lead to your rounds impacting the barricade between you and the target. At best, this can lead to inaccurate shots on target, and at worse it can lead to debris or ricochets coming back at the shooter from the barricade. This is also where a proper standoff between the barricade and the shooter is important.
So what does this mean in real life? Well...
Don't laugh -- this happens all the time when you are not aware of your offset. Keep it in mind as you are training.