http://www.xssights.com/faqs.html#faq4FAQ # 4
AR-15/M-16: Rear Sight Elevation Shifts and the need for a Same Plane Aperture Sight
As originally designed the AR-15/M-16 sight system adjusts for windage at the rear assembly and for elevation in the front to zero the rifle to the individual shooter. To obtain the ballistic adjustment needed for longer range shooting the original rear sight had one aperture higher than the other. While this system would not give the shooter an exact elevation adjustment for a specific range, it would under battle conditions put the bullet close enough for government work so to speak.
The new A2 rifles and carbines employ a rear sight system that includes a range cam to allow the shooter to more precisely adjust the ballistic arc to their needs. This system eliminated the need for an elevation shift between the large close quarters aperture and the smaller long-range aperture. However the aperture shift is still with us today.
Comparing some of the rear sights available today there appears to be 3 variations available. The 1st style (and oldest) has a .014 offset. This gives a 2.52 inch shift in point of impact at 100 yards between the two apertures. The 2nd style has a .017 offset, which shifts point of impact 3.06 inches at 100 yards. The 3rd style has a .024 offset giving a 4.36 inch shift in P.O.I.
All of the points of impact shifts shown above are for the standard barrel AR-15/M-16 that has a sight radius of 20 inches. The shorter sight radius (14.5 inches) of the M4 carbine and weapons of similar configuration increase the amount of point of impact shift. The shifts for these shorter sight radius weapons are 3.48, 4.25, 5.96 inches respectively.
The only reason for the various shifts would be for changes in the ammunition used by the military over the course of this weapons employment.
Most people are completely unaware of this elevation change that occurs when the aperture is shifted from one to the other. They just figure that their rifle is doing something strange.
Our aperture sight for this weapon has both the large and small apertures on the same plane so there is no difference in the point of impact when you shift from one to the other. We have also offset one of the apertures by .007 to make up for the sideways movement of the aperture on the windage screw as it pivots.
By installing our same plane rear aperture you now can sight in your rifle using the smaller aperture and shift back and forth between the two apertures as your light and range needs change without having to do mental gymnastics to know where your bullets impact will be.
and you can flip to the larger aperture for closer range zero
Yes.I have my A2 sight zeroed (per IBZ) with the small aperture at 8/3-2. In order to be on target at the same range with the large aperture, I have to go to 8/3. That's in increase in elevation
1. Sight should be at 8/3 -2 clicks, that is, all the way down, not up a click. Please note removable handle sights are
marked 6/3 (rather than 8/3); also some are in ‘half-clicks’ as well. There should be 3 clicks between 3 and 4 on the
knob. If there are 6 clicks then the sight needs to be set at –4 clicks (instead of –2).
2. Small aperture, nose to firing handle weld.
3. Distance is 50 yards.
4. Point of aim should be point of impact of bullet.
7. Remember you're adjusting the FRONT SIGHT for elevation, not the rear, and that each click is about 1/2"
(actually a little more) at 50 yards. You won't get it closer than that. Don't frustrate yourself trying.
8. You're done. Leave the sight in this position for 99% of your shooting.
9. If you have to shoot targets you KNOW are 300 meters away or more, just click to the right number on the sight.
10. If you're patrolling, set the sight to 8/3 and snap the aperture forward to 0-2. This will provide the same
trajectory as above but with a larger, easier to see thru rear sight. Use this setting if you also have the M68 mounted
as it's quicker to transition to if the sight fails. [Editor's Note - there is some variance with the offset of the A2
aperture - they SHOULD be a 2 click difference - however some manucatures produce them with larger offsets.
Setting the sight to 3 then flipping to 0-2 might now work for your AR. Check it at the range, you want the group to
be centered at 50y, you might need to set the sight at 3 +2 or even 4 to get the large aperture to be correct]
is incorrect.and you can flip to the larger aperture for closer range zero