In many shooting forums, the topic of a zero had come up for the AR-15. I have a set way I zero the rifles for all students. This topic will cover why I use my process, but the 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 using simple, easy-to-understand wording. But this is an in-depth article. You have been warned.
Definitions (in my words) for this article:
- Zeroing: 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 bore line.
- Sight Alignment: The center tip of the front sight post should be placed in the exact center of the rear aperture.
- Sight picture: the image your aiming devices give you superimposed onto the target.
- Line of sight: the visual plane you use to gain sight picture and alignment.
- Accuracy: describes the closeness of the sum of bullet prints on the target about the point of aim.
- Cone of fire: The combined effects of the weapon and ammunition's ability to cluster shots at a distance. Measured in IPHY, MOA, MIL, inches, or Metric. (Precision of the total system)
- Hold off: The act of physically moving the weapon's point of aim to a known direction to gain bullet print on a target.
- Angle of departure: A bullet fired from an accurate horizontal barrel begins to slow down and fall towards the earth immediately upon leaving the weapon's muzzle. 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 (usually written 5.4 moa; see "moa" below). Various NATO armies have recently 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 one milliradian, which is about 0.0573° or 3.43775 moa. In Britain, the term angular mil generally refers to the milliradian. One milliradian corresponds to a target size of 10 centimeters at a range of 100 meters or 3.6 inches at 100 yards. Remember, these MIL measurements are 1/10th of the central unit of measure. EXAMPLE: MIL = 3.6" @ 100 yards is 1/10th of a yard (36").
- MOA: an acronym for "minute of angle," 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, one moa is nearly equal to a target size of 1 inch at 100 yards; in fact, one moa = 1.04720 inches at 100 yards or 10.4720 inches at 1000 yards. In metric units, one moa = 2.9089 centimeters at 100 meters.
- IPHY: an acronym for "inches per hundred yards minute of angle," that is, a term used to explain American-style adjusted scopes that will move the reticle 1 inch at 100 or 2.42 centimeters at 100 meters per 1 IPHY adjustment made to the optic (not true moa)
Ok, so here we go.
When a projectile is shot, it moves down range. Depending on the orientation of the weapon, the projectile might or might not hit the target being aimed at.
The world of carbine use is designed around accurate Muti-shots and unknown distances with a need for terminal performance. Now, we have guidelines to follow regarding the goal of the carbine system. Within these guidelines, we find the limits of how we will use the carbine and how to set the sights to give us the best chances of hits, close and long ranges.
Cone of fire
Using the cone of fire takes all the significant variables and places 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 stricter cone of fire than a rack-grade carbine shooting standard ammunition.
To have the line of sight, bullet trajectory, and point of aim intersect at a designated distance, you must adjust the angle of departure of the carbine on two axes. You change the departure angle for the vertical and horizontal axes by adjusting the carbine's sight device.
As proven in the above video, a bullet fired from an accurate 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), 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 device with an angular measurement unit. This devise can adjust in MOA, MIL, or IPHY.
Any of these units of angular measurement gives us the ability to adjust our aiming device (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 per-click optic will move the departure angle 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 if 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. 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 per click 0.130875". If you shoot 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 25 yds is in a perfect world; we know that we see groups even at 25 yds, and many factors come into play for these groups, mostly having different points of aim per shot. But the idea of a 25-yard zero allowing you to gain hits at 300 yards is not 100% true. You will have some bullets hit the target, but most will miss. None will have accuracy and precision. The same applies to a 50yd zero or distance less than 100 yards. The distance of 100 yds should be the shortest distance you can 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 bullet's trajectory is similar to the flight of a football. When you pass a football at a distance, the football goes up (ascending branch) and then reaches a peak (summit). The football travels downward (descending branch) to the receiver's hands. A bullet in flight follows a path in the atmosphere the same way.
Distance Zero
On a zero set past the bullet trajectory summit, the bullet will cross the line of sight twice. The first crossing is the short-range zero, and the second is the long-range zero. Due 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 200-yard, 250-yard, or 300-yard line is selected due to the guidelines listed at the start of this article.
Any of these distance zeros (200yds, 250yds, 300yds) will keep the bullet's trajectory (flight path) within a human-size target 40" tall and 20" wide with little to no hold-off for the bullet to print on target. We will look at these distances and see how they help us.
The numbers below exemplify shooting XM193 at 2900fps with a mechanical sight offset of 2.5". Remember that the numbers reflected below are the center of the cone of fire, not the exact trajectory each bullet will follow. The generic trajectories are:
200yd zero
- 0yds: 2.5" below the line of sight ( same as mechanical sight offset)
- 25yds: 1.5" below the line of sight
- 50yds: 0.1" above the line of sight
- 100yds: 1.5" above line of sight (summit)
- 150yds: 1.5" above line of sight (summit)
- 200yds: ZERO
- 250yds: 3.5" below the line of sight
- 300yds: 8.5" below the line of sight
- 350yds: 16.5" below the line of sight
250yd zero
- 0yds: 2.5" below the line of sight ( same as mechanical sight offset)
- 25yds: 0.4" below the line of sight
- 50yds: 0.8" above line of sight
- 100yds: 2.8" above the line of sight
- 150yds: 3.5" above line of sight ( summit )
- 200yds: 2.7" above line of sight
- 250yds: ZERO
- 300yds: 4.7" below the line of sight
- 350yds: 11.9" below the line of sight
300yd zero
- 0yds: 2.5" below the line of sight ( same as mechanical sight offset )
- 25yds: .2" below the line of sight
- 50yds: 1.5" above line of sight
- 100yds: 4.4" above the line of sight
- 150yds: 5.9" above line of sight (summit)
- 200yds: 5.8" above line of sight
- 250yds: 4.0" above the line of sight
- 300yds: ZERO
- 350yds: 6.4" below the line of sight
Now, these charts are great when you learn what they tell you. The value I used is in inches about the line of sight.
How this all comes together
Let’s assume we aim for the center of target mass. The above numbers are the center of the trajectory for a carbine’s cone of fire. Now, your carbine, if it's a rack-grade system, let's also assume it's three moa accurate. XM 193 ammunition is loaded to 2 moas 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 300 yds.
This tells us the cone of fire at 100 yards will be 5.235" for extreme grouping. We find the middle value of this extreme grouping radius 2.617" at 100 yards. (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 100 yards of a 200yd zero ( 1.5" high ), the cone of fire of 2.617" extreme radius gives us a value we can work with.
Targets Shot at DPRC on 1 Nov 2012 rifle:
- Barnes Precision Machine AR-15
- Ammunition: M855
- Optic: EOTech G23 FTS 3x Magnifier Gen II w/ Flip To Side Mount
200yd Zero 100yd target
200yd Zero at 200yd Target
200yd Zero at 200yd Target no 3x magnifier
200yd Zero at 300yd Target
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 play on how and where your bullets print on the target. This is why a complete understanding of cone of fire is the best way to maximize the accurate muti-shots, unknown distance with a need for terminal performance your carbine can perform for you.
Join us for your next training day!
John Boyette