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 to a fixed datum. 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 in where your bullets will strike. If not zeroed, your bullets may strike too low, too high, or to the left or right. Shooting with an unzeroed sight is like attempting land navigation with a broken compass. Effects are not obvious over short distances but accuracy is diminished dramatically at medium and long ranges.
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.
The following are some terms and definitions used when discussing ballistics. 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.
Near and Far Intersects (Near and Far Zeros)
When an object is launched forward and at an upward angle, it will follow a parabolic path. This is true for baseballs, footballs, rocks, and it is true for bullets. While the path of a bullet may be parabolic, your line of sight is linear. Therefore, whenever a bullet is fired at an upward angle with respect to the sights, there will usually be one or two intersects. The Near Intersect (Near Zero) is the first time the bullet passes the sight line. The Far Intersect (Far Zero) is the second and final time the bullet will pass the sight line.
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. For most purposes, the HOB of stock sights on a handgun is negligible.
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). For example: from 0 to 300 yards, you may decide that an acceptable deviation might be 2-3 inches. Therefore, the MPBR would be 300 yards. Shorter MPBRs generally have less vertical deviation. Conversely, longer MPBRs generally have more vertical deviation.
As it relates to aiming, Hold refers to the act of aiming high or low to account for the vertical deviation of bullet impact. You are probably familiar with aiming high to account for the effects of gravity, but some distances may require you to aim low.
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” means that the POA and POI are at the same spot.
Minute of Angle (MOA)
Minute of Angle is an angular unit of measure common to ballistics. It is often used to determine how many “clicks” will 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.
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 of the MOA in inches. Therefore, since 1 MOA is about 1/4 inch at 25 yards, multiply the 3 by the 4, and you get 12.
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 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. 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.
(It should be called Muzzle Speed, because Velocity implies both speed and direction)
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 effected by wind 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.
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.
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.
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.
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.
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
Absolute Trajectory Zero 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 off as range increased. Stock handgun sights use this zero. This is not a practical zero for rifles, but it is an option in our search for a better zeroing method.
Corrected Trajectory is any zero that orients the barrel and sights such that the bullet crosses the sight line one or two times. Another method for Corrected Trajectory would be to have the bullet’s parabolic trajectory converge at one point along the site line. This would render 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.
This is not a very practical zero due in part to the difficulty with which it would be calculated. However, it is moving us towards the concept of orienting the barrel with respect to the sights. Another method for Corrected Trajectory would be to have the bullet cross the sight light twice.
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
The 36 Yard Zero has a more exact secondary intersect at 300 yards than the 25 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 exhibits about 5 inches of vertical deviation (plus or minus) from 0 to 200 yards, and a -5 inch drop at 350 yards.
50 Yard Zero
The 50/200 yard zero is popular with Law Enforcement because it provides minimal vertical deviation from 0 to 200 yards. It is unlikely for LE to need to shoot a threat at a greater distance than 200 yards. Plus, the minimal vertical deviation enable greater accuracy when taking high risk shots.
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.
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.
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 Zero Targets.
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.
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.
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.
Zeroing red dots is as simple as adjusting the turrets or dials to shift the impact. Red dot sights typically have markings to indicate how for 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.