There are quite a few issues to consider when zeroing a weapon. We will study these first.
The first issue that must be realized is that a bullet fired from a firearm doesn't travel in a straight line. Instead, it travels in an arc and also drifts to one side. We discussed why this happens and also defined two terms called windage and elevation during our study of sight basics. Therefore, if a bullet strikes exactly the point we're aiming through the sights, from a distance of (say) 50 meters, it will definitely not strike the point that we're aiming at, from a distance of 100 meters (assuming the sights aren't adjusted for elevation and windage) because of the way the bullet moves. Hence, when we say that a firearm is zeroed, there must be a reference distance to the target, at which the sights are zeroed. Since different weapons have different shooting ranges, the reference zeroing distance depends on the type of weapon and the ranges it is typically expected to be used. For instance, pistols may be zeroed at 15 meters or 25 meter range, but an M16 rifle is typically zeroed at 200 or 300 meter range. Even though most pistols can be fired beyond 25 meters and the M16 can shoot well beyond 300 meters, the reference zeroing distance is the distance at which it is typically expected to be used under most conditions.
The second issue is to note is that different manufacturers of cartridges may make propellants which produce different amounts of energy and the profiles and weights of the bullets may be slightly different from manufacturer to manufacturer. These could also affect the path of travel of the bullet. Hence, when the firearm is zeroed, it is normally zeroed with the type and brand of ammunition that it is most expected to be used with.
The third issue that affects where the bullet strikes is based on the individual and the angle that he/she holds their head and peers through the sights, when aiming the weapon. For instance, many rifle shooters place their head so that the tip of their nose barely touches the rifle stock. Since different individuals have different shaped heads, therefore a weapon that has been zeroed for one individual will not necessarily strike the same point if another person is aiming the weapon.
The fourth issue is that as there is wear and tear in the firearm, it may shoot differently after some use, due to wear of the rifling, chamber etc. Rough usage may also misalign the sights. Hence, all weapons will need to reset their zero after some use.
With that said, we will discuss a procedure to zero the iron sights of an M16 rifle (or its civilian variant, the AR-15). The same basic principles can be used for zeroing any type of pistol or rifle for any type of sight.
For an M16 or AR-15, the US Army and US Marine Corps advocate the reference distance for zeroing the rifle to be 300 meters. The actual procedure is done at 25 meters though, using a target that has everything scaled by 1/12th actual size.
The M16 has aperture type iron sights, which we studied about previously. Both the front and rear sights are adjusted in discrete incremental amounts. Each time they are adjusted by one increment, a click sound is heard. Hence, adjustments are specified in number of click steps.
Initially, the user adjusts the sights so that the rear sight is using the larger aperture hole (the 0-2 aperture) and turns the windage knob so that the windage indicator is centered about the windage scale, as shown in the image below. The elevation knob in the rear sight is then set to the 8/3 setting (300 meter setting mark. We will discuss why this setting is labelled "8/3" at the end of this article.) and then clicked one click clockwise past it initially. This rear elevation knob will not be touched again until the adjustments are completed.
Next, the front sight post is adjusted. Note that the front sight is mounted on a flat disc with four notches cut on its edge. There is a spring loaded detent that enters one of these notches and locks the sight in place. To lower or raise the front sight post, one must depress the detent with a special tool (or a sharp object, such as a nail or a bullet tip) and then rotate the sight post clockwise or counterclockwise as needed. Each rotation of 90 degrees produces one click. The front sight is raised or lowered until the base of the front sight post is flush with the front sight well.
At this point, the rifle is said to be set at "mechanical zero" initially.
Next, the weapon is taken to the range and placed on a firm base, such as a bench rest. In the case of an M16, there is a special zeroing target sheet that can be placed at 25 meter distance away from the rifle. The target sheet is appropriately scaled to simulate a target at 300 meter range. The zeroing target sheet is shown below:
The target sheet has a black silhouette with a circle in it. This is the part that the user is supposed to shoot at. There are also eight circles around the black silhouette, each one topped with an arc with an arrow head on one side of the arc. These eight circles indicate which sight to adjust (front elevation or rear windage knob) and the arrows indicate the direction to turn the sight adjustment (clockwise or anticlockwise) to adjust the shot towards the center of the target. Notice that there is also a grid in the target sheet and there are numbers that are on the edge of the grid. These numbers tell the user how many clicks to turn the front sight elevation or rear sight windage knobs. The numbers on the X axis are for how many clicks to turn the rear sight windage knob and on the Y axis are for how many clicks to turn the front sight. On an M16, turning the front elevation sight by one click moves the impact point of the bullet up or down by one grid square. Turning the rear windage knob by three clicks moves the impact point of the bullet by one grid square left or right.
The user initially mounts the rifle on a bench rest (or tries to get as stable a shooting position as possible, using a prone shooting position, sandbags for support etc.) and aims for the center of the black silhouette and fires 3 shots. The user then walks up to the target and examines it and sees where the three bullet holes are and determines their average center. The user then looks at the grid and determines which sights to adjust and how many clicks to adjust them by. Let's say the bullets impacted 5 squares to the right and 2 squares down from the center of the target. Then the user knows from the grid markings that he has to adjust the front elevation sight by 2 clicks clockwise and rear windage knob by 15 clicks anticlockwise. Then the user walks back to the bench rest, turns the sight adjustment knobs appropriately and then shoots 3 more times. Then the user walks back and examines the three new holes and sees if they are hitting the center of the silhouette now. If not, the user again looks at the edge of the grid and determines which sights to adjust and by how much and repeats the procedure until the center of the silhouette is hit to the user's satisfaction.
Now the user turns the rear elevation knob in the rear sight one click counterclockwise, back to the 8/3 setting (the 300 meter setting mark. Remember that it was turned one click clockwise past the 8/3 setting initially). At this point, the rifle is fully zeroed at 300 meter range. To shoot at different ranges, the user now merely needs to turn the rear elevation knob to the appropriate range setting.
The following video shows how the procedure is done:
Of course, this procedure simulates the 300 meter range by using a scaled target at 25 meters. Hence, any small deviation at 25 meters may become more prominent at 300 meters. Therefore, some precision shooters adjust their sights as above, but then take their weapon out to an actual 300 meter range and test to make sure it shoots correctly there also and make minute adjustment changes as needed. Then they set the rear elevation knob to different range settings, such as 400 meters, 500 meters etc. and fine tune the settings appropriately. This way, any small errors will be minimized at longer ranges and negligible at shorter ranges.
Incidentally, this might be a good place to explain why the M16 rear elevation control says "8/3" for the 300 meter setting. If you look at that knob, its minimum setting corresponds to the "8/3" mark. The 3 means the setting for shooting at 0-300 meter range. Now, if the knob is turned 3 clicks, the next visible digit is 4, which is for shooting at approx. 400 meter range. Three more clicks brings the next digit 5 for shooting at approx. 500 meter ranges, the next digit is 6 for 600 meters, the next is 7 for 700 meters and then when the knob is turned further, it completes one full rotation and comes back to the "8/3" setting, which is now the setting for the 800 meter mark, the max. elevation setting on the M16A2. This explains why that mark is labelled "8/3". On M4 rifles, the corresponding knob is labelled "6/3" because the max. elevation setting on an M4 is for 600 meter range.
Since accuracy is affected by wear and tear, temperature, vibration and shock, rough usage (i.e. the sights get knocked out of alignment) etc., it is necessary to re-zero the weapon periodically. How often this is done depends on the user, or organization procedures. For example, different branches of military forces have different requirements on how often to zero weapons.