In many shooting forums the topic of a zero had come up for the AR15. I have a set way I zero the rifles for all students. This topic will cover why I use my process, but the overall idea is for you the shooter to learn a process and how I explain things I use at Trace Armory Group. I try to explain concepts in simple easy to understand wording. But this is an indepth article, you have been warned.
Definitions (in my words) for this article:
Zeroing: is the process of adjusting the sights to ensure the shooter’s Point of Aim coincides with the bullet’s Point of Impact at a chosen distance.
Mechanical offset: the height / distance your sights are offset from the line of bore.
Sight Alignment: Placing the center tip of the front sight post in the exact center of the rear aperture.
Sight picture: the image your aiming devices give you superimposed onto the target.
Line of sight (LOS) : the visual plane you use to gain sight picture and /or sight alignment.
Accuracy: describes the closeness of bullet print on the target to the point of aim.
Cone of fire: The combined effects of the weapon and ammunition’s ability to cluster shots at distance. Measured in IPHY, MOA, MIL, inches or Metric. (Precision of the total system)
Hold off: The act of physically moving the weapons point of aim to a known direction to gain bullet print on a target.
Angle of departure: A bullet fired from a true horizontal barrel begins to slow down and fall towards the earth immediately upon leaving the muzzle of the weapon. Increase the angle of departure (elevating the muzzle) to counter the effects of gravity and allow the bullet to reach further distances.
MIL: A unit of angle measure, used in the military for artillery settings. During World War II the U. S. Army often used a mil equal to 1/1000 of a right angle, 0.1 grad, 0.09°, or 5.4 arcminutes (often written 5.4 moa; see “moa” below). More recently, various NATO armies have used a mil equal to 1/1600 right angle, or 0.05625° (3.375 moa). In target shooting, the mil is often understood to mean 0.001 radian or 1 milliradian, which is about 0.0573° or 3.43775 moa. In Britain, the term angular mil generally refers to the milliradian. 1 milliradian corresponds to a target size of 10 centimeter’s at a range of 100 meters, or 3.6 inches at 100 yards. Keep in mind, these MIL measurements are 1/10th of the main unit of measure. EXAMPLE: MIL = 3.6″ @ 100yds that is 1/10th of a yard (36″).
MOA: an acronym for “minute of angle,” that is, for the arcminute. This unit is commonly used in target shooting to express the angular size of targets or the spacing between marks on a reticle (the grid of lines seen in the eyepiece of a rifle). By coincidence, 1 moa is very nearly equal to a target size of 1 inch at 100yards; in fact, 1 moa = 1.04720 inches at 100 yards or 10.4720 inches at 1000yards. In metric units, 1 moa = 2.9089 centimeters at 100 meters.
IPHY: an acronym for “inches perhundred yards minute of angle,” that is, a term used to
explain American style adjusted scopes that will move the reticle 1 inch at 100yds or 2.42 centimeters at 100 meters per1 IPHY adjustment made to the optic (not true moa)
Ok so here we go.
When a projectile is shot it moves down range, dependent on the orientation, of the weapon the projectile might or might not hit the target being aimed at.
In the world of carbine use, this is a world designed around accurate Mutishots, unknown distance with a need for terminal performance. Now we have a guideline to follow on the goal of the carbine system. Within these guidelines we find the limits of how we will use the carbine and how setup the sights to give us the best chances of hits, close range and long range.
One of the known techniques is the Cone of fire. The use of the cone of fire is to take all the big variable’s and place them on a vertical plane. The variables are muzzle velocity, bullet ballistic coefficient and the inherent precision of the carbine. The more consistent every round you shoot the tighter the cone of fire will become within the abilities of the weapon platform. A precision rifle with match grade ammunition will have a tighter cone of fire, than a rack grade carbine shooting standard ammunition.
Cone of fire
Line of sight, bullets trajectory and the point of aim intersect, to have the line of sight, bullets trajectory and the point of aim intersect at a designated distance, you must adjust the angle of departure of the carbine on two axes. The vertical and the horizontal, how you adjust the angle of departure is by adjusting the sight devise of the carbine.
Diagram of EO Tech Adjustment
As we all know, a bullet fired from a true horizontal barrel begins to slow down and fall towards the earth immediately upon leaving the muzzle of the carbine. If we increase the angle of departure (elevating the muzzle) and adjust the horizontal plane (windage) what happens is we counter the effects of gravity and align the orientation of the trajectory to allow the bullet to reach further distances. The key is to have an aiming devise that has a unit of angular measurement. This devise can adjust in MOA, MIL or IPHY.
Any of these units of angular measurement gives us the ability to adjust our aiming devise (optic or fixed sights) to allow the line of sight to intersect with the point of aim, and the bullet’s trajectory to all meet up at a set distance, and make a repeatable prediction of the bullet’s flight path at other distances.
With you adjusting the sight system and the adjustments having a value ( MOA, MIL, IPHY ) the longer the shot the more accurate the adjustments will be. At 200yds a 1/2moa perclick optic will move the angle of departure by 1.047″ in any axis. This is a lot easier to see a .224″ bullet print move and the grouping of the rounds. If your center of a grouping is 4″ low, 5″ left at 200yds that is an adjustment of 3 to 4 clicks up and 4 to 5 clicks right to center the grouping to the point of aim.
Short range zero: The issue with any short range zero is not the distance you pick, it’s the false idea you have a zero in the first place. Let’s look at the 25yd zero and any 1/2 moa adjusted reflex sight. Moa at 100yds is 1.047″ in diameter. So at 25yds a moa is 0.26175″ or ¼ the diameter ( because you are ¼ closer) this means your 1/2 moa adjusted optic is moving perclick 0.130875″. If you are shooting a .223/5.56mm the bullet diameter is .224″. This means you will have to make 1.7 clicks to see the amount of adjustment to equal the size of a bullet hole.
The above example of the clicks at 25yds is in a perfect world, we know that we see groups even at 25yds, many a factor come to play for these groups mostly not having the same point of aim pershot. But the idea of a 25yd zero allowing you to gain hits at 300yds is not 100% true. You will have some bullets hit the target, most will miss. None will have accuracy and precision. Same is true for a 50yd zero or any distance less than 100yds. The distance of 100yds should be the shortest distance you zero a carbine at in any caliber. The idea of a short range and a long range both providing precision and accuracy with the line of sight is 100% true as long as you do not zero at the summit of bullet flight. AKA 100yds.
Summit Zero: A bullets trajectory is similar to the flight of a football, when you pass a football at distance the football goes up (ascending branch then reaches a peak (summit), now the football travels downward (decending branch) to the receivers hands. A bullet in flight does a path in the atmosphere the same way.
100yd zero
Distance Zero: On a zero set past the bullets trajectory summit the bullet will cross the line of sight twice the first crossing is the short range zero the second crossing is the long range zero. Do to the accuracy / precision of the weapon / sighting system it is best to zero at a distance. You can pick any distance you want. For most general uses the 200yd, 250yd or 300yd line is selected do to the guidelines listed at the start of this article.
Any of these distance zeros (200yds, 250yds, 300yds) will keep the bullets trajectory (flight path) within a human size target 40″ tall 20″ wide with little to no hold off for the bullet to print on target. We will look at each of these distances and see how they help us.
Distance zero
The numbers below are an example of shooting XM193 at 2900fps with a mechanical sight offset of 2.5″ keep in mind the numbers reflected below are the center of the cone of fire, not the exact trajectory each bullet will follow. The generic trajectory’s are:
Distance

100yd ZERO 
200yd ZERO

250yd ZERO

300yd ZERO

0 yards

2.5″ Below LOS 
2.5″ Below LOS

2.5″ Below LOS

2.5″ Below LOS 
25yds

1.7″ Below LOS 
1.5″ Below LOS

0.4″ Below LOS

.2″ Below LOS 
50yds

0.7″ Below LOS 
0.1″ Below LOS

0.5″ Above LOS

1.5″ Above LOS 
100yds

ZERO 
1.5″ Above LOS

2.8″ Above LOS

4.4″ Above LOS 
150yds

0.5″ Below LOS 
1.5″ Above LOS

3.5″ Above LOS

5.9″ Above LOS 
200yds

2.4″ Below LOS 
ZERO

2.7″ Above LOS

5.9″ Above LOS 
250yds

5.7″ Below LOS 
3.5″ Below LOS

ZERO

4.0″ Above LOS 
300yds

10.7″ Below LOS 
8.5″ Below LOS

4.7″ Below LOS

ZERO 
350yds

17.5″ Below LOS 
16.5″ Below LOS

11.9″ Below LOS

6.4″ Below LOS 
used is in inches in reference to line of sight.
How this all comes together: Let’s assume we aim center of target mass. The above numbers is the center of the trajectory for a carbine’s cone of fire. Now your carbine, if a rack grade system, lets also assume its 3 moa accurate, XM 193 ammunition is loaded to 2moa at 300yd precision standards. We add the value of the carbine and ammunition; we have a 5 moa constant carbine able to hold 5 moa of precision out to 300yds.
This tells us the cone of fire at 100yds will be 5.235″ for extreme grouping. We find the middle value of this extreme grouping radius 2.617″ at 100yds. (Worst mechanical deviation). Now you the shooter can pull a shot of the projected path, these are not factored in the cone of fire. If we look at the projected flight path at 100yds of a 200yd zero, ( 1.5″ high ) the cone of fire of 2.617″ extreme radius this gives us a value we can work with. So some bullets could print 1.117″ below the line of sight.
 Targets Shot at DPRC on 1 Nov 2012 rifle
 Barnes Precision Machine AR15
 Ammunition:M855
 Optic: EOTech G23 FTS 3x Magnifier Gen II w/ Flip To Side Mount
Same figuring can be done for a 250yd zero at 100yds ( 2.8″ high ) – (2.617″ radius) = .183″ above line of sight ( best case low round )
As you can see, the total precision of the carbine and ammunition added together, plus the distance you zero at, the projected trajectory of the bullet due to the distance you zero. All come in to play on how and where your bullets print on the target. This is why a full understanding of cone of fire is the best way to maximize the accurate mutishots, unknown distance with a need for terminal performance your carbine can perform for you.