Zeroing and Point of Impact

What is a Firearm Zero

A firearm zero is the relationship between your sights and the impact of a bullet at a given distance. Obtaining a zero is analogous to taring (zeroing out) a scale. Zeroing is the process of adjusting the sights so the Point of Aim is the Point of Impact at a given distance.

Why It's Important

Without a proper zero, you cannot be confident where your bullets will impact. They may impact too low, too high, or to the left or right of where you are aiming. Shooting with an un-zeroed gun is like attempting land navigation with a broken compass. The effects of navigating with a broken compass are not obvious over short distances, but accuracy is diminished dramatically at medium and long ranges

The consequences for shooting an improperly zeroed gun vary depending on the context. These consequences are a mere inconvenience at the shooting range; however, it could be the difference between life and death during self-defense or defense of others. You could miss the threat entirely, or worse hurt or kill an innocent bystander. 

Refer to the images below (Figures 1, 2, and 3) which illustrate how various ranges affect the difference between your Point of Aim and Point of Impact (The Reticle in the images is from an EoTech). In Figure 1, we have a classic hostage shot at close range where the Point of Impact of the bullet is much lower than the Point of Aim for the reticle (more on the reason for this later). In Figure 2, we have a target that is approximately 500 yards away and the Point of Impact is lower than the Point of Aim. And finally, in Figure 3, the Point of Aim and Point of Impact are the same for this 200 yard shot (i.e. the bullet will strike in the center of the reticle).

Note: These deviations are based on a 50/200 yard zero for 5.56mm/.223cal ammunition and will vary depending on several factors that we will discuss in this article. 

Figure 1: Point of Impact BELOW Point of Aim (This shot would have hit the hostage)
Figure 2: Point of Impact BELOW Point of Aim (This shot would have impacted near the target's legs)
Figure 3: Point of Impact = Point of Aim (The bullet would impact in the center of the reticle)

Ultimately, it’s your responsibility to ensure your guns are zeroed properly and to understand where your bullets will impact. Understanding the relationship between your Point of Aim (Center of your reticle/sights) and Point of Impact (where the bullet hits) is crucial if you want to hit your intended target. 

Contrary to popular belief and what you see in video games and movies, the bullet’s impact is not always where the sights are aimed. This is because the path of a bullet is parabolic (curved), but your line of sight is linear (a straight line). 

Given that the sight-line is linear and the bullet’s path is parabolic, we must adjust the sights so that the trajectory of the bullet intersects the sight-line at a known distance (or two known distances as we’ll discuss later). And these points of intersection are known as your “Zero”.

Disclaimer

This blog post DOES NOT constitute or form an instructor-student relationship between the author (me) and the reader (you). Online education can only supplement formal instruction, it cannot replace it. It is your responsibility to understand the firearm laws in your area and to follow the Firearms Safety Rules.

Terms

First we need to define some terms relating to ballistics and zeroing. It’s not necessary to fully comprehend these terms. However, a cursory overview will aid in understanding the mechanics of zeroing. 

Maximum Ordinate (Ballistic Apogee)

Maximum Ordinate is the highest vertical travel of a projectile before it begins to fall back towards the earth. When you throw a ball to someone and toss it upward, the Maximum Ordinate is the height the ball gets before it begins to travel back down. See Figure 4 below. 

 
Zeroing and Apogee
Figure 4: Path of a projectile.

Near and Far Intersects (Near and Far Zeroes)

When an object is launched forward and at an upward angle, gravity will cause it to follow a parabolic path (think: parabola). This is true for baseballs, footballs, rocks, and it is true for bullets.

Whenever a bullet is fired at an upward angle with respect to the sight-line, there will usually be one or two intersects. The Near Intersect (Near Zero) is the first time the bullet passes the sight line as it travels upward. The Far Intersect (Far Zero) is the second and final time the bullet will pass the sight line as it is traveling back down to earth.

Bullet Aerodynamic Lift?

This upward trajectory of the bullet is not due to any sort of “lift” imparted on the bullet. It is due to the barrel being angled upward relative to the sights (more on this later). 

However, under certain conditions a phenomena known as the “Magnus Effect” can occur. The Magnus Effect is when Lift occurs to a spherical or cylindrical object in a flowing fluid due to a pressure differential caused by unequal wind resistance on the top and bottom of a spinning object. 

In Figure 5, we have a rotating cylinder submerged in water. The cylinder is rotating counterclockwise and the water is flowing from right to left. Imagine if we were able to observe two individual water particles as they flowed around the object. And suppose one of the particles went over the cylinder, while the other particle went under the cylinder. 

The particle that went over the cylinder would be flowing in the direction of rotation with the cylinder, which would cause the particle to speed up. This increase in flow velocity would also cause a pressure drop. 

However, the particle that flowed under the cylinder would be flowing against the rotational motion and this would result in the particle slowing down. This decrease in flow velocity would also cause a pressure increase. 

Therefore, as we sum up all the billions of particles flowing over the top and under the bottom of the cylinder, we are left with what is known as a pressure differential. We would have more pressure exerted upward on the cylinder than downward, and this would cause Lift. 

This same principle applies to bullets. When a gun is fired, the rifling of its barrel imparts a spin on the bullet. And if there is a crosswind, the Magnus effect can push the bullet up or down, depending on the wind direction, wind speed, and rotation of the bullet. 

HOWEVER, for the purposed of Zeroing, this is does not concern us and the Magnus Effect as it relates to bullets is more for precision shooting. 

Figure 5: Magnus Effect on rotating cylinder submerged in flowing water.

Height Over Bore

Height Over Bore (HOB) is the distance between your firearm’s bore axis and the center of your reticle. A larger HOB means that the sight line is much higher and further away from the barrel. A smaller HOB means that the sight is lower and closer to the barrel. The benefits and tradeoffs of having a higher or lower HOB are beyond the scope of this article. Also, the HOB of factory sights on a handgun is negligible. See Figure 6.

Figure 6: Height Over Bore

Point Blank Range

Point Blank Range does not necessarily mean really close range. Point Blank Range is the distance where your point of impact is within an acceptable Maximum Vertical Deviation from the point of aim. What is acceptable is situational dependent on your context. See the animation below for an illustration of Maximum Vertical Deviation.

Maximum Point Blank Range (MPBR)

MPBR is a method of zeroing to ensure your vertical deviation of impact does not exceed a specified amount over a given distance (See animation below)Notice how the Maximum Vertical Deviation between the bullet’s flight path and the sight line do not exceed the width of the imaginary “cylinder” for a given range. The length of this “cylinder” represents the Maximum Point Blank Range

For example: from 0 to 300 yards, you may decide that an acceptable deviation is 2-3 inches. Therefore, the MPBR would be 300 yards with a 2-3 inch deviation above or below the sight-line. 

Generally speaking, shorter MPBRs generally have less vertical deviation. Conversely, longer MPBRs generally have more vertical deviation.

Point of Aim and Point of Impact

Point of Aim (POA) is where you are placing the center of the reticle or dot. Point of Impact (POI) is where the round actually impacts the target. The phrase “Point of Aim Point of Impact” often means that the POA and POI are at the same spot.

Hold Over/Under

Depending on the range to the target and the relationship between the bullet’s trajectory and sight-line at that distance, you may need to aim high (Hold Over) or aim low (Hold Under). This is probably the most unintuitive concept and one that often confuses many students. Especially when they hear that close ranges require a hold over (aiming over the target) and some mid-ranges may require a Hold Under (aiming under the target). When the POI  is much lower than the POA, this requires a Hold Over (aiming high) to account for the POA/POI deviation. Subsequently, when the POI is much higher than the POA, this requires a Hold Under (aiming low). See Figures 7, 8, and 9.

Figure 7: Point of Impact BELOW Point of Aim (Correct Hold Over)
Figure 8: Point of Impact BELOW Point of Aim (Correct Hold Under)
Figure 9: Point of Impact = Point of Aim (No Hold Over/Under Needed)

Minute of Angle (MOA)

Minute of Angle is an angular unit of measure common to ballistics. It is often used to determine how many sight adjustments are needed to move the impact a certain distance on the target. 1 MOA represents about 1 inch at 100 yards, 2 MOA represents about 2 inches at 100 yards, and so on. (Note: The exact conversion from MOA to Inches is 1 MOA = 1.047 in at 100 yards.)

Some shooters get confused when making adjustments at less than 100 yards. If you are making adjustments at, say, 50 yards, 1 MOA will be about 1/2 inch. At 25 yards, 1 MOA is about 1/4 inch. So, if you need to move the impact 3 inches at 25 yards, you will need to move the impact 12 MOA. 

An easy calculation method is to multiply the distance you want to move the impact by the denominator (number on the bottom of the fraction) of the MOA in inches. Therefore, since 1 MOA is about 1/4 inch at 25 yards, if you want to move the impact by 3 inches, simply use: 3 x 4 = 12 clicks. 

(Click Factor) = 100 / (Distance to Target in Yards)

Number of Clicks = (Click Factor) X (Desired Impact Shift in Inches)

Variables

Many variables affect your zero and determine what zero is appropriate for your context. Some variables are external, some relate to the firearm and optic, and some relate to the shooter.

Muzzle Velocity

Muzzle Velocity is the speed a bullet exits the barrel. The time it takes a bullet to travel from the bore to the target is a function of Muzzle Velocity, bullet mass, bullet geometry, and air resistance. Generally speaking, faster Muzzle Velocity means less flight time over a given distance than a slower Muzzle Velocity

When a bullet is in flight between the bore and the target, gravity will act on it and pull it to the ground. If you zero with a cartridge with a certain Muzzle Velocity and then shoot another cartridge with a different velocity, your zero will be off. Therefore, ensure you zero your firearm with the cartridge you plan to use. And always confirm your zero with new ammunition. 

Nerd Note: It should be called Muzzle Speed, because Velocity implies both speed and direction. Therefore, if you fire the same rifle towards the North and then again towards the West, it’s Muzzle Velocity has changed but it’s Muzzle Speed is the same.

Bullet Mass

The inertia of a bullet is partially determined by its mass. A bullet with more mass requires more force to accelerate as well as decelerate. The acceleration occurs inside the chamber and barrel as the hot gases expand from the burning gun powder. The deceleration occurs as the air slows down the bullet. 

Bullets with higher mass have more kinetic energy than bullets of lower mass when traveling at the same speed. Now, gravity accelerates both bullets at the same rate, regardless of their mass. However, the bullet with the higher mass and more kinetic energy will be less affected by air resistance, meaning it will not decelerate as quickly as the bullet with lower mass and lower kinetic energy. Therefore, the kinetic energy will factor into bullet flight time and bullet flight time will determine how long gravity is permitted to act on the bullet.

You might be tempted to think that bullets with more mass will travel higher than bullets with less mass. However, this is not the case. Suppose you fired two bullets of the same size and shape but with different masses directly upward at the same velocity in a vacuum (no air resistance). Though the heavier bullet will have more Kinetic Energy than the lighter bullet, each bullet will still reach the same altitude before falling back down. Now, if this were to occur with air resistance, then the bullet with more mass (i.e. more Kinetic Energy) would travel higher than the bullet with less mass.

Wind

Wind exerts a force on the bullet as it travels to the target. A head wind will cause more resistance in the direction of travel and make the bullet decelerate more quickly. A tail wind reduces drag and can create a shorter flight time. Finally, cross-winds generate lateral shift left or right. When zeroing, it is best to do so with minimal or no wind factor. As with gravity, the longer the flight time, the more time wind has to effect the bullet.

Assuming no wind factor, your windage adjustment (left/right) should be the same for any distance. So, with no wind, the horizontal deviation of your Point of Impact should not deviate at any distance. Unlike the elevation (vertical) deviation, which is influenced by gravity. 

Temperature

Temperature factors into air density; creating either more or less drag on the bullet. Atmospheric temperature also effects the cartridge temperature, influencing the temperature of the powder, which effects chamber pressure and muzzle velocity. A firearm zeroed on a day with temperatures in the teens will not have the same zero on a day with temperatures in the nineties. Consider zeroing for each season or climate you plan to operate in.

Skill

If you cannot apply the Fundamentals of Marksmanship, you cannot zero a firearm. Zeroing requires repeatability by the shooter. If you cannot maintain a good grouping (relative placement of bullet holes to one another), then you have no basis to adjust your sights. Zeroing requires a trend of shot placement. If there is no trend, then there is no way of knowing the true relationship between the bullet’s trajectory and the sight line.

Patience

Even if you have skill, you need the patience to take the necessary time to zero and confirm your zero. You might be precise during the first few shots, but if your grouping becomes sloppy, you have lost your basis for adjusting your sights. Obtaining and maintaining sight pictures requires meditation. If while you are waiting for the shot to break, you loose focus on your sight picture, take a break and reset. 

Do not rush through a zeroing session. 

Zero Variations and Their Application

Before you select a particular zero, you must first consider the application you intend for the firearm. The following Zeroes will pertain primarily to Armalite (AR) variants chambered in 5.56 or .223.

Absolute Trajectory Zero

An Absolute Trajectory Zero (shown below) orients the sights or optic such that the sight line and bore axis are parallel. For close ranges, the round would impact below the aiming point and then impacts would drop lower and lower as range increased. Stock handgun sights use this zero. This is not a practical zero for modern rifles, but it is an option in our search for a better zeroing method.

Corrected Trajectory

A Corrected Trajectory Zero (shown below) is any zero that orients the barrel at an upward angle relative to the sights. This upward angle launches the bullet upward so that it crosses the sight line one or two times. This would cause a lower impact at close ranges, a point of aim point of impact at the peak of the parabolic arch, and then a lower impact after that point

Known Distance Zero

A Known Distance Zero would be zeroing the sights so that the point of aim point of impact converged at a predesignated distance. This might be used if someone only plans to engage at a given distance for a hobby or competition.

25 Meter Zero

The 25 Meter Zero was adopted by the U.S. Army. It has a near intersect at 25 Meters (Approximately 27 Yards) and a far intersect at 300 Meters (Approximately 328 Yards). I suppose the use of meters instead of yards was to aid with cross-over with NATO. 

The round will impact just under 5 inches high at 100 meters, about 5.5 inches high at 200 meters, and 5.5 inches low at 350 meters. This zero provides the greatest Maximum Point Blank Range (MPBR). Although, it also causes the greatest vertical deviation within it’s MPBR of nearly +- 6 inches.

36 Yard Zero

This zero is what we used in the Marines, and I believe it is still used. The 36 yards was determined after evaluating the ballistics of the 5.56 NATO rounds fire through an M16A4 Service Rifle. This zero offers a very long Maximum Point Blank Range (MPBR), depending on your optic height. 

50 Yard Zero

The 50/200 yard zero (more accurately a 50/250 yard zero) is popular in Law Enforcement because it provides minimal vertical deviation from 0 to 300 yards. It is unlikely they’ll need to shoot a threat at more than 300 yards. The minimal vertical deviation enable greater accuracy when taking high risk shots. A logistical benefit of this zero over the 36 yard zero is that most ranges have a 50 yard firing line, whereas a 36 yard firing line would need to be measured manually. 

With a 5.56/.223, you can expect a Point of Aim Point of Impact at 50 yards and 200 yards, with a maximum vertical deviation of +- 2 inches within 200 yards. At 250 yards, the vertical drop is about 3 inches, and at 300 yards, the drop is about 6 inches. 

This zero enables the shooter to aim at center mass of a humanoid target from 0 to 300 yards, and strike within a combat effective region. The ability to aim center mass and not have to “hold over” helps during a stressful shooting.

100 Yard Zero

The 100 yard zero causes the bullet to have a point of aim and point of impact at 100 yards. For most calibers, this means that you will have low impact under 100 yards, a higher impacts between 100 yards and the point at which the bullet drops back under the sight line, then lower impacts beyond.

The images below (Figures 10, 11, 12, and 13) compare the Point of Impact deviation from one zero distance to another for a .223 Remington with a Muzzle Velocity of 2,850 [fps], a Sight Height of 3.25 [in], and Bullet Weight of 70 [gr]. The targets used are IDPA style targets. It’s important to note that these deviations are not exact and will vary depending on bullet weight, Optic height, and several other factors; however, these approximations are good enough for our purposes.

You’ll notice that some zero distances offer a longer Maximum Point Blank Range (MPBR) on a human sized target, but it’s a balance of MPBR with how much deviation is acceptable for those intermediate ranges. Regardless of the zero distance you choose, it’s important to know the Over/Under impacts of your bullets for a given distance with that zero.

Figure 10: POI deviation for 25 Yard Zero
Figure 11: POI deviation for 36 Yard Zero
Figure 12: POI deviation for 50 Yard Zero
Figure 13: POI deviation for 100 Yard Zero
Figure 14: These were the parameters used to calculate the Point of Impact deviations used above. These are estimates and may not reflect your ammunition. Calculator can be found here: https://shooterscalculator.com/ballistic-trajectory-chart.php

How It's Done

Fundamentals of Marksmanship

Before you can zero a firearm, you must be able to apply the fundamentals of marksmanship. If you cannot maintain a consistent shot grouping, then it is impossible to adjust your shots. It is possible to use a bench rest or vice to hold the firearm in place and these make zeroing even more precise. However, having a zeroed rifle will not help if you cannot apply the fundamentals.

Targets

Various targets exist for zeroing and most of them accomplish the same thing. At a minimum, a Zero target should consist of a grid so that you know how many adjustments are needed vertically and horizontally. Some Zero targets account for Point of Impact off-set so that you can Zero at shorter distances if you do not have access to a long range and to mitigate the effects of wind. If you use a Zero target with off-sets, you should always confirm the Zero. Check out our free Zeroing Targets or you can scan the QR code below:

Iron Sights

Iron Sights consist of a front and rear sight. The front sight usually has adjustments for elevation (vertical shift). The rear sights usually have adjustments or both elevation and windage (horizontal shift). The distance between your front sight and rear sight is called sight radius. Sight alignment is generally easier when there is a longer sight radius. Aiming is basically drawing lines and whenever you are drawing a line between two points, the more distance between the points, the better.

Front Sight

Elevation adjustments (vertical shift) during zeroing should only be performed using the front sight. This is because most rear sights have Bullet Drop Compensation (BDC) adjustments on their dial. The BDC should be set to whatever distance you are zeroing at and then all vertical adjustments performed by moving the front sight post up and down. When adjusting the front sight post, moving the sight post up will cause the bullet to impact lower. Conversely, moving the sight post down will cause the impact of the round to shift up.

Rear Sight

The rear sight’s elevation should be set to the respective zeroing distance. Moving the rear sight right will move the impacts right, and moving it left will move impacts left. The windage should be adjusted as needed and then the windage knob should be marked with a paint pen to indicate zero. The zero marking with help you return to true windage zero if you ever need to adjust for actual wind conditions.

Red Dots

Zeroing red dots is as simple as adjusting the turrets or dials to shift the impact. Red dot sights typically have markings to indicate where the impact will shift. Adjustments are commonly measured in Minutes of Angle (MOA). Read the MOA explanation in the Terms section above.

Context is King

When determining what Zero works best for you, consider what the purpose of the specific firearm (home defense, hunting, law enforcement, military, etc.). There are advantages and disadvantage to each type of zero. If you don’t expect to engage targets beyond 15 yards, your zero may look much different than one used for 0 to 300 yards. Regardless of what Zero you use, the most important thing you can do is to train and practice engaging targets are various distances with that Zero. 

Additional Resources and References

Until next time...

Train To A Higher Standard!

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