In order to explain, what is zeroing the AR-15 rifle, we must address a few aspects associated with trajectory of bullet path, gravity, line of sights, impact point. Trajectory of bullet path fired from the rifle is a path resembling the path of thrown stone. The difference is only in the size of bullet, reached speed and covered distance in the time. The cartridge fired from the rifle placed horizontally to ground will fall through the gravity affecting it, in perfect conditions, with the same speed of falling as the cartridge dropped from the hand will fall vertically down. It is not important, what speed has the bullet, because it will fall due to gravity.
Therefore, in order to shoot the target on determined distance we must throw, and in case of weapon to fire a bullet at certain angle. Line of sights is a straight line connecting the sighting point and our eye with target point, which we want to shoot. It is possible to imagine that this is laser beam led from our eye to final target. This image is not affected by gravity, so it is a straight line.
Shooting with a rifle we have the following overlapping phenomena. Cartridge fired out from rifle barrel, which is directed at a right angle from the ground to reach designated by us bullet path. Rifle scope through which we derive the line of sights, begins from our eye, and ends on the final target, in which we want to shoot. The scope is additionally placed over the barrel of 4-7 cm, what requires an adjustment on our part to perfectly hit the target.
Therefore, in order to shoot the target on determined distance we must throw, and in case of weapon to fire a bullet at certain angle. Line of sights is a straight line connecting the sighting point and our eye with target point, which we want to shoot. It is possible to imagine that this is laser beam led from our eye to final target. This image is not affected by gravity, so it is a straight line.
Shooting with a rifle we have the following overlapping phenomena. Cartridge fired out from rifle barrel, which is directed at a right angle from the ground to reach designated by us bullet path. Rifle scope through which we derive the line of sights, begins from our eye, and ends on the final target, in which we want to shoot. The scope is additionally placed over the barrel of 4-7 cm, what requires an adjustment on our part to perfectly hit the target.
And now famous Zero, what it is actually? It is simply a distance from the operator of rifle to the point connecting curved trajectory of bullet path with straight line of sights.
Depending on the Zero chosen by us i.e. distance e.g. 25 m, 50 m, 100 m or 300 m, the bullet can twice cut the Zero line. At first rising above the line of sights, and then while falling under the line of sights. Therefore, zeroing the rifle simply means configuration of sights (permanent or optical) in such a way to perfectly cross the line of sights at determined distance with the bullet path. For sniper shooting, at long-distance reach, there are additional difficulties in identifying the target point such as: Coriolis force, deviation of bullet path by the wind, type of bullet, its weight and initial speed, as well as the way of its behavior on air flow resistance. However, in this description we will focus only on AR-15 rifle with ammunition of 5.56 mm /.223 Remington 55 grain and initial speed of 988 m/s, which was designed for short-distance action in the range of 0 m - 300 m.
In order to let you understand the determined target points, we have created the outline depicting two obtained Zeros on the Trajectory of bullet path by adjusted sights on 50 m Zero. Adjusting our devices (the optical centre of our scope is 7.5 cm on the barrel) on 50 m we will hit the target on this distance, but if the target avoids hitting on this distance, then parameters of bullet path will cause that the same placed target at a distance 250 m will be hit without changing our way of aiming and parameters of our equipment. The image of path is presented on the scheme.
Now when we have clarified all concepts, what setting the Zero in our rifle will be best for us?
Assuming that AR-15 rifle will be used to combat firing on a short distance, usually the maximum distance doesn't exceed 200 m, and occasionally is up to 300 m. Our goal is to set the Zero, i.e. our sights, to achieve the best focus in a full-scale of these distances without additional amendments of sights. Therefore, we have created two types of firing and target areas, Combat Effective Zone and Target Effective Zone. In combat situation (Combat Effective Zone), extreme and stressful, the perfect Zero will be such which without any amendments will enable neutralization of the target. In accurate firing of disc (Effective Zone Target), the perfect Zero will enable to hit the target of diameter 10 cm (i.e. area smaller than Combat Effective Zone) on different distances without any amendments.
Therefore, is there a perfect solution to set the Zero on AR-15 rifle in such a way to hit the target in a full-scale without any amendments? Unfortunately, there is no perfect solution, but establishing above descriptions of our shooting target i.e. Combat Effective Zone and Target Effective Zone we are able to find the optimal solution. Below are presented schemes for values 25 m, 50 m, 100 m, and 300 m. In each scheme is a reference point of hits for different distances without additional amendments. Colorful points indicate different distances from the operator on the bullet path and the same colors refer to individual direct hits on the target placed on the right side of scheme.
If you set the Zero on 25 m:
Combat Effective Zone in a range of 0m - 490m and the dispertion of hits
is in a quite great scope, thus our possible mistakes can cause a significant
reduction in efficiency
Target Effective Zone in a range of od 0m - 50m
For Zero set for 50m :
Combat Effective Zone 0m - 360m
Target Effective Zone 0m - 300m
For Zero set for 100m:
Combat Effective Zone 0m - 320m
Target Effective Zone 0m - 220m
For Zero set for 300m:
Combat Effective Zone 0m - 400m
Target Effective Zone 0m - 110m
Assuming in addition that by setting the Zero on a determined distance we must to such a distance shoot the rifle through and theoretically every time we will take off the optical devices we should repeat the process. Whereas taking into account the fact of quite difficult access to shooting ranges of 300 m, setting the Zero on this distance will be troublesome. During dynamic trainings, very rarely you will train at a distance of more than 100 m, and only from time to time on 200 m.
Considering our target i.e. obtaining the best Combat Effective Zone, and the best focus Target Effective Zone. If you want to get a compromise of all mentioned parameters, as well as at the same time taking into account the purpose of AR-15 rifle and parameters obtained by cartridge 5.56 mm /.223 Remington, obviously in the first place is Zero at a distance of 50 m. In our tests we have used the collimator placed quite high 7.5 cm on the axis of barrel, and therefore the second Zero at adjusted distance of 50 m appears on 250 m. However, the standard optical copes and their assemblies don?t exceed the value of 6 cm on the axis of barrel and then with setting 50 m Zero, we have the famous second Zero at distance of 200 m ? hence the formula 50 m/200 m, i.e. by setting the Zero on 50 m we have at the same time the second Zero to a distance of 200 m.