Zeroing question

[dave43]

I zeroed my AR15 at 50 yards. I used the small aperture. When I switched to the large aperture I shot inconsistent groups....all were low. What am I doing wrong?

[Molon]

0-2

http://www.box.net/shared/static/jmpyln5i8o.jpg



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.

http://www.box.net/shared/static/4sfcffz4gw.jpg


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.


http://www.box.net/shared/static/wptq3nw8ww.jpg





The shift 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.http://www.ar15.com/images/smilies/icon_smile_big.gif)

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
http://www.box.net/shared/static/4kpcgxyskw.bmp

[Mac5.56]

Well that was an awesome and very informative response. Thank you for taking the time to publish this, I have learned a lot about the large aperture that I didn't know. This is why I love this site!!!

[DaBears_85]

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.

Interesting read. My only experience is with BUIS, Troy to be more specific, as far as AR's are concerned. I always thought that the 0-2 was for targets 0-200m and the smaller was 300-600m. I must admit I'm a bit of a greenhorn when it comes to irons but am working on it.

[markm]

You're not doing anything wrong. When I try to shoot the large app, even at silly close range, I can't hit a damned thing.

I'm better off shooting with my eyes closed than the large app.

[arizonaranchman]

Outstanding post Molon, thanks for the info. I always shoot with the smaller aperture, but when just carrying the rifle around the ranch I keep the large aperture flipped up as mtn lions are my primary concern and a quick response for that type of threat makes the large aperture a good choice.

If I have any time to do so for a shot I always switch to the smaller aperture, but keep the large one up routinely.

I use Ashley/XS sights on my Marlin lever rifles and like them alot.

[az doug]

XS sights does sell a same plane rear sight that does not have a shift when you flip from small to large aperture. I know several LEO's that use them because of the distances they normally shoot at.

[Mac5.56]

You're not doing anything wrong. When I try to shoot the large app, even at silly close range, I can't hit a damned thing.

I'm better off shooting with my eyes closed than the large app.

Coming from you that makes me feel better. The first time I tried to walk and shoot I used the large aperture thinking that it would be the best option. I missed every shot on a 12 Pepsi box. Thought it was a pretty big indication that I sucked horribly. Now I realize after this thread that I was probably hitting way below the target itself.

[m4fun]

i remember the NCOs were all up in arms about the big hole on the A2s and were ready to swap them out with the A1 rear appature. When my unit got A2s, nothing formal about it, just better, longer range, etc., although it was an engineering company. I never remember using the big hole.

Excellent post Molon!

[ForTehNguyen]

great just when I had all my rifles zeroed using the small aperture. At least that explains why the smaller aperture on magpul MBUS is elevated slightly. So if I have midlength rifles I should turn down the front sight by 4.25 MOA or ~4 clicks?

[az doug]

I have a set of MBUS on a SBR I took to a 40 hr patrol rifle course recently. I have an Aimpoint on it and seldom use the back up sights. I normally only zero with the large aperture.

In this class they had us zero with the small aperture. It was only then that I noticed the small aperture was at 11:00 o'clock and not 12:00 o'clock on the leaf. I knew the small and large aperture were not on the same plane, but Magpul did not place the hole centered at on the leaf of the small aperture. I will probably remove the leaf with the small aperture and zero with the large aperture.

Someone told me the new MBUS only have a large aperture. I do not know if this is true or not. The person that told me usually keeps up with those sort of things.

I just checked Magpul's 2010 catalog and the MBUS Gen II will be dual aperture, so it appears my friend is wrong.

[ForTehNguyen]

i turned my MBUS front 4 clicks down with corresponded with the cowitnesses on my eotech when using the large aperture. 4 clicks down sounds like a lot to move but I guess if it cowitnesses. For sure I know the Eotech is dead on at 50. I was under the impression that 4 clicks down is almost 8 MOA, but above it said 4.25" @ 100 yards? Getting confused.