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Posted: 1/18/2004 2:38:31 PM EDT
| I have a 20" A2 which is zeroed at 50 yards but through the small peep. How many 1/4 turns on the the front sight post would be needed to change it to zero using the large peep - and which direction to turn? (just trying to same some trial and error zeroing time at the range). |
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The large aperture drops the POI something between 3 and 6MOA, I can't recall which. That is the offset between POI and POA from 0-200 when the rifle is zeroed at 300 using the small aperture. The large ap is the "low light, moving target, close range" sight. If you want a "same plane aperture" those are available. At 50 yards one click on an A2 front (rifle SR) is 0.63" as they're usually 1.25MOA clicks. Given (at most) a 6MOA shift between apertures you should be 3 inches low at 50 so you're looking at between 4 and 5 clicks of Up. |
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Quoted: The large aperture drops the POI something between 3 and 6MOA, I can't recall which. Tweak - I have a link at my site to X.S. Sights FAQ which covers this. They tested 3 different rear apertures and got 3 different results (in the range you specified). Scary! |
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Yep [url]http://groups.msn.com/TheMarylandAR15ShootersSite/sightsopticsfaq.msnw[/url] Q: What is the change of impact between the two apertures in the A2 sight? A: Courtesy of XS Sights from their website (http://www.aosights.com/news.html FAQ #10) ...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 MD-AR article references [url=http://www.aosights.com/faq/10.html]this[/url] at the XS page FAQ#10 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. |
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Forest, Tweak, correct me if I'm wrong on this, but the following was always my understanding of the way the sights work (IBSZ method, of course): You zero at 50 yards with the small aperture and -2 clicks on the elevation. Then, without moving the elevation knob, you flip up the larger (0-2) aperture, and you're good to go from 0-200 yards. For longer distance, you flip up the smaller aperture and turn the elevation drum to 8/3 (for 300 yards) or whatever is appropriate for the range of your target. Close? |
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If you're talking about the Army method it is zero POI/POA at 25M with the small aperture and the rear elevation set to 8/3+1. Returning the elevation to 8/3 after zeroing supposedly results in a 300M far zero and flipping to the large aperture allows for effective engagement from 0-200. However, a 25M zero like this results in headshots at 300M. Are you talking about the COL Santose's 50 yard IBZ? You can change the range of your near zero, change the POI of your near zero, zero at your far zero range, or change the height of the rear sight base then return it to 8/3. But 8/3+1 at 25M doesn't result in the best (most centered) far zero. I just zero low (bottom of the 4cm circle on the A2 zero target) at 25 meters with the sight set at 8/3 and wind up with COM 300M strikes. Forest has an excellent page on this at his site. |
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Quoted: You zero at 50 yards with the small aperture and -2 clicks on the elevation. Then, without moving the elevation knob, you flip up the larger (0-2) aperture, and you're good to go from 0-200 yards. No. 1) Zero with the small aperture at 50. This is a 200M setting for the small aperture - set to '3' for 300, '4' 400 etc. 2) To use the LARGE (0-200) aperture you need to set the small aperture to '3'. Then flip to the large aperture for a 200M setting for it. This is covered in LTC Santose's instructions. The only potential 'glitch' is if you have a large aperture that is lower than the 'normal' (about 2.2 moa with rifle sight radius). In the worst case the sight should still be fine COM shots to 200M (just a bit low). But the large aperture isn't meant for precision - just speed and usability in low light. You can solve the whole issue by spending $33 and get the XS Same Plane rear aperture. |
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