Iron Sight Apertures

[rob_s]

Anyone have measurements for some of the various apertures out there? A1? A2? Troy? Magpul? XS same plane?

ETA, info from below

A2: small = 0.070” | large = 0.200”
XS: small = 0.100" | large = 0.230"
TR: small = 0.075" | large = 0.195"
MI: small = 0.075" | large = 0.200
MP: small = 0.075" | large = 0.190"

[DaBears_85]

Are we talking about the diameter of the apertures?

[rob_s]

Yes, I'm interested in the ID of the apertures themselves. If there is a shift, I'd also be interested in knowing how much of a vertical shift there is in the two aperture centerlines.

[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 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

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



.....

[rob_s]

Interesting read. So...

A2:
small = 0.070” | large = 0.200”
offsets
0.014" = 2.52" shift at 100 yards
0.017" = 3.06" shift at 100 yards
0.024" = 4.36" shift at 100 yards


Still curious as to A1

XS website (http://www.xssights.com/store/tactical.html)says
small = 0.100" | large = 0.230"

I have heard some people complain that the XS is slightly "too big" so I wanted to quantify that, and that also piqued my interest on some other sights. Still interested in Magpul and Troy. IIRC Larue uses a standard A2 or the XS, and DD uses the standard A2.

[ST911]

I had a table of the IDs of the popular ones, but can't find it. The only one I remember off the top of my head is the 0-2 A2 aperture, at ~.195".

[rob_s]

I would have sworn I had seen a similar table or chart but can't seem to locate it now.

[orionz06]

Troy:

Small: 0.075"
Large: 0.195"

[polymorpheous]

Troy:
Large Ap - .195
Small AP - .075

Midwest Industries:
Large Ap - .200
Small Ap - .075

Magpul:
Large Ap - .190
Small Ap - .075

[Joeywhat]

DD A1.5 is .070 and .200.

[danpass]

National Match sights for highpower shooting start at .030 (maybe smaller but I've never seen them) and go up.

I started with an .030 but now use an .040


I use an .070 square front post