US Marine Corps 25-Meter Zeroing Procedure
Jimmy Hots jimmyhots@gmail.com 4/14/2009 Introduction When it came time to zero my AR-15, in my naiveté I assumed there existed one standard, universally accepted approach that everyone follows to obtain a good zero. After some investigation, however, I learned that quite the opposite is true. There is not one, but several well-known approaches, and they differ significantly from each other. Front Sight Zero, USMC Zeroing Procedure, and the Improved Battlesight Zero are just three examples of well-known zeroing techniques. Each of these uses its own particular methodology to achieve a zero possessing a unique set of advantages and disadvantages. To make matters more confusing, many more lesser-known zeroing techniques exist, adding to the plethora of potential choices. To give one example, the 12 meter zero is an approach sometimes used by those who expect to work within the context of close-quarter, close-range engagements. This was all quite confusing to me at first, until I started sorting through the details in an effort to understand the differences between the various zeroing techniques, and what the advantages and disadvantages are in choosing each. This is an ongoing learning experience for me, and I think a true understanding can probably come only through much practical experience. However, I have found that by making an effort to understand the underlying physics of the ballistics involved, I am able to answer many questions for myself related to the how’s and why’s behind these zeroing techniques. My goal for the rest of this paper is not to analyze and compare various AR-15 zeroing approaches; I mention them only to put into context the real purpose of this paper, which is to give an account of my experience following the US Marine Corps 25-meter Zeroing Procedure. Even though this will be presented from a practical perspective, a little physics will be included to help make sense of things. So, with that said, let’s start with the physics!
Trajectory Physics The graph below shows the theoretical trajectory for the Federal XM193 55 grain round zeroed at 300 meters (328 yards). This is the trajectory obtained, in theory, by a 300 meter zero such as the USMC Zeroing Procedure. Note that the vertical scale of the trajectory has been greatly exaggerated so that the curve is easier to see. Although the rendering of the curve has been exaggerated, it is still numerically correct. The first thing to notice is that the bullet trajectory does not, in general, coincide with the line of sight. This may be a remedial fact to some but for me it was something that I had not fully considered before. I simply had a fuzzy notion of the bullet more-or-less following the line of sight. There are two primary reasons for this mismatch between the trajectory and line of sight:
There are other reasons why the trajectory does not coincide with the line of sight, but they need not be discussed here.
Another thing to notice in the graph is that the trajectory crosses the line of sight in two places; known as the “cross-overs.” After the bullet exits the barrel of an AR-15, it is below the line of sight because the sights rest above the barrel (in the case of most removable-handle sights the initial deviation is 2.6 inches, for most fixed-handle sights it is 2.8 inches). As the bullet travels it moves towards the line of sight, until it reaches the first cross-over. Note that this upward motion of the bullet is not due to a lift force as is sometimes believed, but is simply the result of the way the rifle is sighted and the resulting (small) angle between the barrel and the line of sight. After the projectile reaches the first cross-over, its trajectory takes it above the line of sight for some distance, until it falls back down to intersect the line of sight at the second cross-over. This second cross-over occurs at a distance that is considered to be the “sight-in” range, i.e. 300 meters for a 300 meter zero. From here the bullet continues to descend until it reaches the end of its flight. The table below shows properties of the trajectory for the Federal XM193 55 grain round zeroed at 300 meters (328 yards). It is an alternative view of the same data shown in the previous graph. The first cross-over occurs at about 30 meters, with the second being at 300 meters (corresponding to a 300 meter zero). The height of the trajectory peaks at 178 meters, with the maximum deviation being 6.55 inches. This means that at the maximum height of the trajectory, the bullet is 6.55 inches above the line of sight.
All of this goes to explain the following. From the muzzle to first cross-over (at about 30 meters), the bullet strike will be low. At about 30 meters, the bullet will hit at line of sight. From 30 meters to 300 meters, the strike will be high, with a maximum high-strike of about 6.55 inches occurring at a range of about 178 meters. At 300 meters (sight-in range), the bullet will strike at line of sight. From 300 meters and beyond, the strike will be low. Note that most AR-15s have a rear-sight elevation adjustment which is designed to alter the trajectory to account for ranges past the 300 meter mark. Everything written above assumes that the elevation knob is kept static at the 6/3 (or 8/3) setting. It should also be noted that the initial deviation (the vertical distance
between the center of the bore and the line of sight at the muzzle) varies
between different types of AR-15s, and this does have an effect on the
trajectory. The data above was calculated using an initial deviation of 2.6 inches because that is approximately correct for my removable-handle rifle. Alternatively, fixed-handle rifles
usually have an initial deviation of 2.8 inches. This results in a
similar trajectory, but the first cross-over now moves out to
36 meters instead of 30 meters, and the maximum deviation becomes about 6.45 inches. The larger 2.8 inch initial deviation yields (approximately) the well-known 37 meter first cross-over for the 300 meter zero. It is interesting to note that the 37 meter cross-over is only valid for rifles with an initial deviation of 2.8 inches. The first cross-over for rifles with an initial deviation of 2.6 inches (i.e., removable-handle AR-15s) with a 300 meter zero is 6-7 meters closer, at about 30 meters. Now that we’ve covered some of the physics behind projectile trajectories, it’s time to look at the USMC Zeroing Procedure.
USMC Zeroing Procedure The USMC Zeroing Procedure is readily available in the operators manual TM 9-1005-319-10 for the Ml 6A2/M4 (http://www.ar15.com/content/manuals/TM9-1005-319-10.pdf) and will not be reproduced in full here. It defines a procedure for zeroing against a target at 25 meters, which will result in a 300 meter zero. The basic steps are as follows:
The A2 25 meter zeroing target is readily available (http://www.bobdbob.com/~deneb/doc/targets/m16a2-25m-zero.pdf); a copy of which is reproduced below.
The target has been scaled here and is not of regulation size. When printed to proper size, the silhouette will be 4 cm across. Note that each quadrant of the target shows the adjustments necessary to bring the group to the center. The numbers along the edges specify the number of clicks of front-sight and rear-sight adjustment necessary to bring the group to dead-center. This is a nice feature and helps to eliminate much of the guess-work and errors involved when obtaining a zero. The A2 target above is meant to be used to zero M16s/AR-15s with 20 inch barrels. For M4 rifles with shorter barrels, the grid lines used to determine the number of adjustment clicks needed to bring the group to center will not be spaced correctly. Thus, the numbers along the perimeter of the grid indicating number of clicks will not be accurate for barrel-lengths other than 20 inches. This is because the distance between the front and rear sights differs with barrel length, and there is a corresponding difference in the point-of-impact shift per click of elevation and windage. The table below shows how elevation point-of-impact is affected for both 16 inch and 20 inch barrels at a range of 25 meters:
The following table shows how windage point-of-impact is affected for both 16 inch and 20 inch barrels at a range of 25 meters:
It can be seen from the tables above that the adjustments are more sensitive on the 16 inch barrel that on the 20 inch. Although the clicks themselves adjust the sights by the same amount for the 16 inch and the 20 inch, they produce a larger effect in the case of the 16 inch because the front and rear sights are closer together. When the A2 zeroing target is printed to proper dimensions (center circle 4 cm in diameter), each grid square is 0.9 cm in height, and about 1 cm in width, which indeed corresponds (approximately) to the adjustment sensitivities for the 20 inch barrel. When zeroing an AR-15 with a shorter (or longer) barrel, the A2 target can be used to produce successful results, if it is kept in mind that the grid markers will not give completely accurate indications of the number of clicks needed to bring the group to center. AR-15s with barrels shorter than 20 inches will yield larger point-of-aim shifts as predicted by the grid markers, and those with barrels longer than 20 inches will produce smaller point-of-aim shifts. Other than what was previously described, there are no differences in the USMC Zeroing Procedure when zeroing AR-15s of varying barrel lengths. The target center circle has a diameter of 4 cm in all cases, the range is 25 meters, and the rear elevation is set to 'z' (or the number of clicks above 6/3 (+2) or 8/3 (+1) corresponding to 'z' setting). There does exist an M4 zeroing target meant to be used for short-barrel M16s/AR-15s, but the only significant difference between it and the A2 zeroing target used here is in the spacing of the grid lines to account for the increased adjustment sensitivity in short-barrel rifles. For my purpose, I determined that the readily-available A2 target would suite my needs perfectly well. I simply payed attention to the fact that increased adjustment sensitivity will have to be taken into account when reading the grid markers. Now that we have reviewed the USMC Zeroing Procedure, it’s time to put these things into practice and unleash the firepower!
The USMC Zeroing Procedure in Practice The rifle used for my implementation of the USMC Zeroing Procedure is my Colt 16” carbine:
To provide a stable platform for the targets, I built a simple target stand out of scrap lumber:
Ammunition used was Federal XM193 55 grain ball military surplus:
Also used interchangeably with the XM193 brown-box was Federal XM193 55 grain loose pack (same ammo, just packaged differently):
A bench with two sandbags provided a stable firing platform:
This is the perspective to the target at 25 meters:
Rear sight elevation was set to ‘z’ per the USMC Zeroing Procedure:
The front sight was adjusted using an A2 front-sight adjustment tool (much easier than using a bullet tip):
Here is the first group. All the strikes are uniformly low:
Before firing the second group, I brought the front sight down two clicks, thus raising the point of aim. Here are the results:
The front-sight adjustment definitely raised the strikes. But it raised them too much; now they are a bit too high. As noted in the section above on the USMC Zeroing Procedure, the A2 target grid line spacing is accurate only for 20 inch barrels. For a 16 inch barrel, the numbers on the grid will suggest too much of an adjustment. In this case, for the first group, the grid numbers suggested a front site adjustment of about 2 clicks down. However, we see in the second group that this over-corrected the point-of-impact due to the increased sensitivity of the 16 inch barrel (compared to the 20 inch). Before firing the third group, I brought the front sight up one click. Here are the results:
The group was lowered, but a little too much. Also, the groups so far tend to be left of center. Before firing the fourth group, I took the front sight down one click, and the rear windage two clicks clockwise. Here are the results:
This one is getting close. Most of the strikes are right of center however, with one straggler. Before firing the fifth group, I adjusted the rear windage one click counter-clockwise. Here are the results:
At this point I felt that things were very close; the center of the group is within the center circle. I felt that the rifle was probably zeroed in, and attributed the spread of the group to operator error, i.e. crappy shooting ;-) I fired another group without adjusting the sights, this time being careful to place my shots. Here are the results:
This group showed all five shots within the circle, which was good enough for me. I called it a successful zero and packed up my gear before the sun went down.
Conclusions and Next Steps The USMC Zeroing Procedure was found to be a very straight-forward and easy-to-follow method for zeroing the AR-15. Unfortunately, I have not yet had a chance to test the true accuracy of the method, having only shot with it at 25 meters. The true test will be to place some shots at 50 meters, 100 meters, and so on up to300 meters to see how the trajectory behaves, and to see if the second cross-over does indeed occur at the expected 300 meters. Note (4/15/2012): Part II has been completed and published: http://sites.google.com/site/jimmyhots/usmc25mar-15zero-part-ii
Addendum
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