I know that I have talked about this before, and I promise you I will talk about it again.
While I was in the Marine Corps we shot a qualification course of fire at the distances of 200, 300, and 500 yards. Using the 8/3 sights of the M16A2 we used the markings on it for 300 and 500, and adjusted it 2 clicks down from 8/3 small gap for 200 yards.
When I got out of the Corps, I found much to my dismay that the carry handle sights I used would bottom out on 8/3 or 6/3 if it was a detachable sight. Turns out they come from the factory that way. The intent is that the small peep is used from 300+ and you would use the larger 0-2 aperture on 6/3 or 8/3 for a 200 yard zero during low light or close range shooting.
Turned out the Marines would modify the sights to allow for a 200 yard zero. And this modification is as simple as loosening a screw.
Now to back track for a moment. On a rifle length AR15, a fixed carry handle with the 8/3 sight will have a 1 Minute of Angle (MOA) change in impact per click of the elevation wheel. The detachable carry handle will have an adjustment of 1/2 MOA. On the carbine, this adjustment is about 3/4 MOA.
So from the factory, the AR rear sight will bottom out on 8/3 or 6/3. We call this small gap.
One full turn puts you on a 800 yard zero on a fixed carry handle, and 600 on the detachable carry handle. We call this the large gap. That size of the gap lets you quickly identify which of those settings the sight is on.
To allow you to set the sight for a 100 or 200 yard zero, you need to allow the drum to rotate below 8/3 or 6/3. You will need a small Allen Wrench. I’ve found that this wrench size is not the same on all brands of carry handles.
When the rear sight peep is up and the sight is aligned on 6/3 or 8/3 , you can insert a small Allen Wrench into a screw.
DO NOT REMOVE THAT SCREW.
Just loosen it a turn or two. This will allow you to rotate the bottom section of the elevation drum.
On a 8/3 drum, -2 clicks gives a 200 yard zero. 8/3 -3 for 100.
For the 6/3 drum, double the number of clicks. -4 for 200, and -6 for 100.
Snug the screw back down, and double check that you have the right number of clicks. Zero your rear sight normally and then you will be able to dial your rear sight down for a 100 yard zero.