Rear sight operation?

[Rastoff]

Here is a picture of a standard rear aperture sight on an AR-15:

Yes, that's a Troy folding sight, but the idea is the same.

My question is this, is the center of the two apertures different?

When I was in basic (almost 30 years ago) they told us the small aperture was for 500 yards or more. So, is it higher as well as smaller? That would make sense because it's for longer distance.

 

[JaPes]

I'm not an expert, but the small aperture acts like a quasi lens. If you wear glasses, take them off. Look at something at distance that looks fuzzy. Make a tiny peep hole with your hand. Shrink the aperture until you can see the object clearly. Your field of view is limited, but you can see that one thing clear.

That the mad scientist in me talking off the top of my head.

 

[cyphertext]

On the standard A2 type system, yes, the center of the small aperture is higher than the center of the large aperture. There are some sights that have "same plane" apertures where the center is the same height. These are not to be confused with "same plane" sight systems that are referring to both the front and rear sight being mounted on rail systems that are the same height, or same plane.

 

[WR Moore]

Answer to the basic question: yes on sights patterned on the A1/A2 sight leaf, the smaller aperature is higher above the bore. On the A2 system, the large diameter aperature is for 0-200 meters, the smaller aperature (and the adjustment wheel) is for 300 plus. The military zero method should be described in your owners manual.

If you're not military, most of us will not be shooting at anything over 200 meters/yards with irons-unless you're shooting NRA Service Rifle matches. That being the case, a 200 m/y zero will work well. A 50 m/y zero will hit zero again at about 225 m/y and give you a good point blank range to 250. I'd suggest using the larger aperature.

As noted, check the information with whatever sights you bought (or check their website) to see if they stick with this system or if the aperatures are on the same plane. If all else fails, shoot a couple groups at, say 200 yards/meters. One group on one target with large aperature, shoot another target with the small aperature and look at elevation.

The A1 system is reeeaaally fuzzy after 40+ years. IIRC, the larger aperature was 0-300 meters, the smaller for beyond 300.

 

[Rastoff]

On the standard A2 type system, yes, the center of the small aperture is higher than the center of the large aperture.

Thanks cypher, that's exactly what I was thinking, but wasn't sure.

I'd suggest using the larger aperature.
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The A1 system is reeeaaally fuzzy after 40+ years. IIRC, the larger aperature was 0-300 meters, the smaller for beyond 300.

At the ripe old age of 47, the large rear aperture has become almost useless. I've devolved to the red dot more out of necessity than anything else. Still, I'd like to know how to use my sights properly if I need/want to.

I'm not an expert, but the small aperture acts like a quasi lens.

The term you're grasping at is depth-of-field. The smaller the aperture (hole) you look through the greater the depth-of-field. This basically means that more stuff will be in focus. When I was shooting in indoor pistol competitions, I'd use an occluder and an adjustable aperture. This helped to block my off eye and the aperture made everything from the rear sight to the target in focus. It was a tremendous advantage for slow fire 25yard competition.

Because it's a small hole, it's a tremendous disadvantage is speed is a requirement. It reduces the field-of-view dramatically.

Ultimately, I'd like to find a smaller rear aperture. Even though it will cost me in target acquisition time, it will help with aiming. Then again, I should probably just stick to the red dot or HWS.

 

[bhayles]

On the standard A2 type system, yes, the center of the small aperture is higher than the center of the large aperture. There are some sights that have "same plane" apertures where the center is the same height.

Its dependent on the manufacturer. Because of this thread, I just called Magpul, as it is their MBUS rear site that came with my S&W AR Sport.

In the case of the Magpul MBUS, both apertures are on the same plane.

 

[gunny4053]

In theory, the two apertures give you a perspective view of comparable sized objects at relative distances. Again, in theory, a man viewed at 100 meters through the large aperture, should appear the same relative height through the small aperture at approximately 300 meters. The small aperture reduces the margin for error as the target progresses further away from the shooter. Once the optimal range is achieved, the the margin for error begins to increase again, just as it did with the large aperture.

Some of today's sights have objective centers (the middle of the apertures hole), the same height from the center of the pivot point. Some have attempted to aid longer range shots, by raising the small aperture, and others, such as Knight, have developed sights with a single size aperture and micrometer adjustable base to raise the sight for a specified range.

With the old M16A1, we essentially had to use the Tennessee elevation method for adjusting for longer ranges. Shooting a 300 meter range with pop up targets at 50, 75, 100, 150, 200, 250, and 300 meters, using the small aperture, we had to shoot low on the 50, dead on at 250, and high at 300 to hit the same relative location on the target. In essence,due to the rise and fall of the bullet through its' trajectory, belly button at 50, center of mass at 250, and the head at 300, in order to place center of mass shots on each target. I believe when we got the A2's or possibly the A3s', the rear sight had an adjustment for elevation/ range on the rear sight.

I could be wrong, it's been 22 years since I retired, 31 years since I was an instructor (Drill Sgt.), and 43 years since I was in basic, so the memory could be fading.

OK, I've spouted my minutia for tonight.

Good Luck and good shooting.