Some Thoughts On Zeroing The AR-15/M16/M4 Rifles.
(This has been updated to show some other zeroing strategies and to correct some transcription errors.)

The current US Military doctrine is to zero at 250 meters (273 yards) which equates to a 25 yard point of impact of .9" low or dead on at 40 yards with the full size M16 and M855 ball ammo.  For the same 250 meter zero for the 14.5 " M4 the 25 yard point of impact is .7" low at 25 yards or dead on at 36 yards.  Some organizations and instructors like other zeroing techniques such as a 25 yard or 50 yard point of impact.  In an effort to see what these zeroing strategies actually do below are the results from numerous runs with ShootingLab using actual recorded velocities (M855 - 2970 and M193 - 3170) from a16" barreled M4 and  the Aberdeen Proving Ground supplied G7 ballistic coefficients of  .151 for the M855 and .122 for the M193.  Note that the nominal sight height is 2.5".  Note the very high mid-range trajectory of the "25 yd zero."  From a full sized M16 the trajectories will be a little flatter.

I have highlighted the approximate 3' drop range as that seems to be the amount that most shooters can determine and suggest that that range be the general maximum engagement distance for effective hits.

Note that all this data is accurate as far as the given input data, (run as sea level standard metro).  However, it is an approximation due to environmental and ammunition variances in the "real world."  But as they say, "It's close enough for government work.

While many claim that the 5.56 isn't effective past about 275 - 300 yards, modern combat experience has show the need to be able to effectively engage targets past those distances, and the enhanced (MK 262/362) rounds have effective ranges way past the M855.  The 25 and 50 yard zeros have the advantage of using dead center hold at the given zero range and of using ranges commonly marked out but the 25 yd zero especially, has a very large maximum ordinate (7.9" inches).  The other zeros require some specific distance from point of aim for the zero at 25 or 50 yards, or the use of a "non-standard" distance for a point of aim zero.  In my mind the "250 meter" zero (39.1 yard near zero) or the 6" vital zone zero 250 yard (40 yard near zero) make the best sense and give a good compromise in hitting close and distance targets with minimal fuss.  Hold COM to 300 yards and past that hold at the top of the head., but to each his own.

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Disclaimer

As far as I know all the information presented above is correct and I have attempted to ensure that it is. However, I am not responsible for any errors, omissions, or damages resulting from the use or misuse of this information, nor for you doing something stupid with it. (Don't you hate these disclaimers? So do I, but there are people out there who refuse to be responsible for their own actions and who will sue anybody to make a buck.)

Updated 2017-09-15