Confusion on the ideal M16/AR-15 zero abound because people don’t understand how their sights work.
Army and Marine doctrine is a 300 meter no wind zero. This point blank setting is often called a BZO or battle sight zero. The elevation markings on the rear sight were made with this zero in mind.
Because this BZO creates a maximum ordinate of about a half foot some people recommend a 200 yard zero as an “improvement”, which can be established using an POA/POI at 50 yards for the initial intersection. Still others suggest zeroing at 100 and leaving it.
A savvy marksman can use any of these zeroes and keep the markings on the drum intact. The M16A2 and newer have rear sight adjustable for windage and elevation with two apertures. The older technical manual (TM 9-1005-319-10, page 47) illustrates this pretty well. The current manual (TM 9-1005-319-10, ZEROING PROCEDURE, section 11, page 01) still shows this but is not as clear.
First, slip the elevation so you have four minutes of angle of adjustment BELOW the 300 meter setting. This is four clicks with a M16A2 or A3. Note the 300 meter setting is the rear sight all the way down with the “3” centered on the port side. Rotating this up a full revolution (25 minutes of angle) goes from 300 to 800 meters. When the sight is all the way up there is an “8” indicating the 800 meter elevation setting. It is marked 8/3 but is not “eight thirds” (or is it a compound fraction 2-2/3?) as non-shooters call it.
Detachable issue sights, as found on the M16A4 and M4, are half minute per click. Given 25 clicks for a complete revolution, this yields only 12.5 MOA above 300 meters. It takes 12 minutes (3 + 4 + 5) to go from 300 to 600 meters so the sight is marked to 600 (6/3) This detachable sight should be slipped for elevation to have eight clicks (four minutes) below the 300 meter setting.
Typically, a 200 meter zero is two MOA below 300 and 100 meter zero is one MOA down from 200.
One MOA above 300 meters is POA/POI at 25 meters, so initial zeroing can be done there. A 200 meter zero (2 MOA below 300) is POA/POI at 50. 100 meter zero is 3 MOA below 300. With a properly adjusted rear sight the markings are still correct.
AR-15.com has picture tutorials on how to do this:
http://www.ar15.com/forums/topic.html?b=3&f=18&t=328143
http://www.ar15.com/forums/t_3_18/328143_.html
When you have 4 MOA of adjustment below the 300 meter setting you can zero dead on at 200 or 100. Go 2 MOA down from 300 for a 200 meter zero and 3 MOA down from 300 for a 100 meter zero. Click to 300 for a doctrine BZO. Zero at 25 meters set 1 MOA above the 300 meter setting.
There are two rear sight apertures. Zero with small peep sight. With a 300 meter BZO flip to the larger aperture marked 0-2. On issue sights this larger aperture has its center lower than the smaller aperture, thus lowering your point of impact, in effect creating an “improved” zero for closer range shooting. Some after market sights are machined to have the centers of both aperture at the same point. If you don’t know, shoot a group with both apertures to see what POI change is present.
Firearm User Network 
Jan 30, 2013 @ 01:14:16
You might want to go into some detail about maximum ordinate for those folks who don’t know the term. Also, how it applies to the radius of the intended target height above center of mass when determining where your point-blank or battlesight zero should be set.
Lots of folks don’t know about this stuff.
LikeLike
Jan 31, 2013 @ 12:54:18
The operating environment should be taken into consideration when deciding what zero to use. Where are the likely engagements predicted to occur? What are the distances involved? What are the rules of engagement / use of force policies in effect? Taking these and other factors into consideration, in the city where I work a 50 yard zero works best. A more rural area might require a 300 yard zero. This is a great topic.
LikeLike
Jan 31, 2013 @ 20:25:21
TM, a good general-purpose zero is possible using the point blank method. This uses the height of your intended/anticipated target as measured from dead center to about one or two inches below the top edge of the target. Zero distance is placed at a range where aiming dead center on the target at the distance of maximum bullet rise above the line of aim (the maximum ordinate) will not result in the bullet rising enough to miss over the top of the target. Beyond the actual zero distance, the bullet will be dropping below the line of aim, yet for a short distance, it will still hit the target below dead center when aiming dead center. This distance out to just before the bullet falls below the bottom edge of your intended target, is maximum point blank range. Further out than this, you have to compensate.
This method allows the furthest possible distance to zero the rifle and capture the greatest percentage of possible engagements into the situation of “simply hold center and squeeze for a hit”. It simplifies your decision-making process in the most possible instances – all you have to decide is whether the target is within your max PBR. Yes? Hold center and squeeze. No? Make necessary compensation. You control the decision of how tall your intended target will be and how high your maximum ordinate will be, and where that translates into a zero distance.
One good thing about this method is that it helps avoid unecessary fiddling with your sights. Making mistakes there results in misses.
LikeLike