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Posted: 5/17/2010 7:04:00 PM EDT
0-2
There seems to be some confusion around here as to the function of the “0-2” large aperture of the A2 rear sight. The field manual for M16A1 and M16A2 Rifle Marksmanship states, “The larger aperture, marked 0-2, is used for moving target engagement and during limited visibility. The unmarked aperture is used for normal firing situations, zeroing and with the elevation knob for target distances up to 800 meters. The unmarked aperture is used to establish the battlesight zero.” That’s pretty straightforward. Using a digital caliper, the diameter of the small aperture of the A2 rear sight measures approximately .070”. The large (0-2) aperture has a diameter of approximately .200”. No confusion there. To use the 0-2 large aperture, simply push forward and down on the small aperture portion of the sight leaf to snap the large, 0-2 aperture up into place. That couldn’t get much easier. The confusion surrounding the use of the 0-2 large aperture seems to stem from a misunderstanding of the amount of elevation change involved when changing from the small aperture to the large aperture. The small aperture is intended to be zeroed for a 300 meter zero using M855 fired from a 20” barreled M16A2. The 0-2 large aperture is intended to provide a 200 meter zero, when the small aperture has been properly zeroed with M855 from a 20” barreled M16A2. If you examine the trajectories of M855 fired from a 20” barreled M16A2 zeroed at 300 meters, and at 200 meters, you will see that the difference in elevation between these two trajectories (and hence the difference in elevation between the small and large apertures) is 2.5 MOA. 
To demonstrate this concept, I fired two 10-shot groups from a 20" Colt AR-15, using the standard A2 iron sights, from the bench at a distance of 50 yards using the small aperture. I then flipped the rear sight to the 0-2 large aperture and fired another two 10-shot groups (all groups were fired on separate targets.) The two sets of groups were overlayed on each other to form 20-shot composite groups using RSI Shooting Lab and analyzed to determine the statistical center of their points of impact. In a perfect world, the groups fired using the 0-2 large aperture would have the center of their points of impact 2.5 MOA (1.25” at 50 yards) below the center of the the points of impact of the groups fired using the small aperture. The 20-shot composite group formed from the 2 groups fired using the 0-2 large aperture had a center of point of impact 1.01” below the center of point of impact of the 20-shot composite group fired using the small aperture. 1.01” at 50 yards is 2.02 MOA; not exactly 2.5 MOA, but within a half minute of angle and well within my margin of error using the A2 iron sights. As a side note, the groups fired using the 0-2 large aperture were 144% larger than the groups fired using the small aperture. 
The shifts will definitely be greater with carbines due to the shorter sight radius. I’d like to thank Hallorann for sending me a link to the XS Sights (formerly Ashley Express) website where there is some information that may shed some more light on this subject. I have not experienced this myself and therefore cannot vouch for the validity of the information, but according to XS Sights there are three different A2 rear sight leafs on the market that have different elevation offsets; however, XS Sights does not state which manufacturers/models use which rear sight leafs. (My experience with the A2 sights has been strictly with Colt 20" models and they have all had the 2.5 MOA offset for the 0-2 large aperture, and yes, I drank the Kool-Aid.  )
According to XS Sights: “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.” The last statement in the above quote seems rather odd to me, as M855 still is, and always has been the standard issue ammunition since the adoption of the M16A2. Also, I don't see any particular use for the 3.06 MOA and 4.36 MOA offsets for a 20" barrel. With the small aperture properly zeroed for 300 meters, the 3.06 MOA offsett will produce a 165 yard zero and with the 4.36 MOA offset, the bullet will never cross the line of sight. 20" barrel with M855 zeroed for 300 meters 
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| Molon, thanks for the great information. I am a relative newb to the AR platform and have a 20" Bushmaster with a detachable carry handle and A2 sights. Right now I have it zero'd using the M-16 A2 25m zero and your post is helpful for those trying to understand how the sights work. |
| Interestingly, my Daniel Defense M4 a1.5 sight has the 0-2 aperture printing higher than the unmarked when the unmarked is zeroed for 50/200 yds. This is the opposite of what it should be but then again the DD sight is really a bastard sight having the a2 apertures but an a1 mechanism with no elevation knob for use with the small aperture. |
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I read an article a couple of years back where the author measured a number of A2 sights and found a wide range of differences in the height of the aperture center.
Did you experience any change in horizontal POI between the two settings? Another issue addressed in the same article was the lateral movement of the site on the threaded mount. |
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If this is addressed to me, the answer is no, both groups were horizontally centered, with the 0-2 aperture grouping being approx. 1.5" higher at 50 yds.
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I read an article a couple of years back where the author measured a number of A2 sights and found a wide range of differences in the height of the aperture center. Did you experience any change in horizontal POI between the two settings? Another issue addressed in the same article was the lateral movement of the site on the threaded mount. |
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I read an article a couple of years back where the author measured a number of A2 sights and found a wide range of differences in the height of the aperture center. Did you experience any change in horizontal POI between the two settings? Another issue addressed in the same article was the lateral movement of the site on the threaded mount. Genuine Colt A2 rear apertures are manufactured with a horizontal off-set to address the lateral movement when shifting between the two apertures. I can't speak to the construction of other brands. The shifts will definitely be greater with carbines due to the shorter sight radius. I’d like to thank Hallorann for sending me a link to the XS Sights (formerly Ashley Express) website where there is some information that may shed some more light on this subject. I have not experienced this myself and therefore cannot vouch for the validity of the information, but according to XS Sights there are three different A2 rear sight leafs on the market that have different elevation offsets; however, XS Sights does not state which manufacturers/models use which rear sight leafs. (My experience with the A2 sights has been strictly with Colt 20" models and they have all had the 2.5 MOA offset for the 0-2 large aperture, and yes, I drank the Kool-Aid. )
According to XS Sights: “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.” The last statement in the above quote seems rather odd to me, as M855 still is (at the time of this writing), and always has been the standard issue ammunition since the adoption of the M16A2. Also, I don't see any particular use for the 3.06 MOA and 4.36 MOA offsets for a 20" barrel. With the small aperture properly zeroed for 300 meters, the 3.06 MOA offsett will produce a 165 yard zero and with the 4.36 MOA offset, the bullet will never cross the line of sight. 20" barrel with M855 zeroed for 300 meters
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I'm still getting used to using irons.(red dot user) I zero with the small aperture at 50 yards. I get good groups. If I switch to the larger 1 my shots open up.
I always thought the difference was for a precision shot you should use the small aperture, & to just shoot on target quickly use the larger one. |
| Molon, fascinating thread. Questions - do we know the origin of the rationale why they programmed in a 200 meter zero? How can we determine that was their intent? makes sense, but is their historical data on this? What was the reason they felt a MOA change was warranted back then? Do different manufacturers maintain this 2 MOA difference? eg, colt, FN both do this? I have called two folks at FN this morning to see what they can tell me about current production carry handle sights on the M-16. |
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Molon, fascinating thread. Questions - do we know the origin of the rationale why they programmed in a 200 meter zero? How can we determine that was their intent? makes sense, but is their historical data on this? What was the reason they felt a MOA change was warranted back then? Do different manufacturers maintain this 2 MOA difference? eg, colt, FN both do this? I have called two folks at FN this morning to see what they can tell me about current production carry handle sights on the M-16. I can only tell you the "what." I'm not about to try to guess "why" Colt and the US Military did what they did. 
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This is good info, I was considering a stand alone sight and the A-2 Aperture made little sense in a stand alone.
I had an Daniel 1.5 before and it shot lower with the 0-200 apet. Maybe I should reconsider it and adjust a 25/300zero with the small and 50-200 with the large. |
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How come I can't even hit the paper at 100 yards with the 0-2 apperture, when I can hit 5" circles from the same distance? It seems that I always hit two or three feet low 
That's with every AR I've owned.  My guess is you might be centering the top of the protective ears instead of the front sight post in your sight alignment. I catch myself doing it sometimes. Posted Via AR15.Com Mobile |
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