I dug these out of the sight parts box, On the left is a National Match type rear sight apeture, in the middle is an original M16 rear sight apeture, and on the right is the improved battlesight apeture used on late production M16A1 rifles and most everything that has followed so far. There are some back up iron sights that use different apetures available now.
This image shows the 0-200 meter close range apetures on the three types of sights;
This image shows the long range apetures of the three types of sights;
One can see there is not a great deal of difference in the sight apeture holes on the A1 sight.
The A1 sight is not designed as a target sight but as a ghost ring battle sight.
We were taught in the Army to regulate the sight at 25 meters.
To do so the shooter flipped the rear sight to the long range apeture and used the front sight post and the windage dial to bring the shot group onto the mini silhouette target.
Once the setting was achieved the shooter had a 300 meter battle sight setting.
This would allow the shooter to flip the rear sight up to the short range apeture and achieve hits on man size,(19"wide-36" tall) silhouette targets
using a center mass aiming point at ranges of 0 to 200 meters.
Flipping back to the long range apeture would allow hits on the same target from 300 to 400 meters.
If I remember correctly, 440 meters was the effective combat range and 1500 meters was the effective range designated for the M16A1 rifles at the time I was in service.
The Army taught us to use the front sight at a point blank hold dead on the area one intended to hit.
The Marines regulated the sights differently because they shot at bullseye and not human silhouette targets.
Their technique taught the shooters to use the bullseye to dot the eye of the top of the front sight post. HTH
