When I started the Appleeed shoot over the weekend, I was a bit frustrated with the whole zeroing process. It’s not that anything Appleseed did, but more about the settings of the AR-15 platform in general. The 6/3 carry handle sight of the DMTR is designed to be zeroed at 25 meters, and then rechecked at 300 meters. This works because the ballistic trajectory of a 5.56 bullet fired from an AR-15 will travel upwards to its maximum point about 100 meters away, and then begin falling back to Earth. The point 25 meters away (when the bullet is traveling up to its maximum), and the point at 300 meters where it’s falling back to earth are roughly the same elevation.
This is all explained in great detail by Molon in a thread over at M4carbine.net
The problem is that because the sight is meant to be zeroed for 300 meters (at the closest) the rear carry handle sight of the AR-15 bottoms out at that setting does not allow for a 100 meter zero, which has a point of impact a good bit bit above a 300 meter setting. The military intended for all personnel to use the 25m/300m zero point, and then swap to the larger 0-2 aperture for fighting (the 0-2 aperture is actually on a different plane than the smaller unmarked sight, which gives a usable BZO). This works great for fast paced run and gun where tight accuracy standards are not required. However, this is not ideal for marksmanship practice where precision at various ranges is important. When I arrived at the Appleseed shoot, my sights were set for a solid 100 meter zero, as thats where I practiced. I could have twisted the rear sight drum (which is 1/2 MOA increments) rather than the front sight post to make a temporary zero for the event, but I figured there had to be a better way. And there is.
Some years ago, when I first got into AR’s, I saw a thread on M4carbine.net about Lt. Col. Chuck Santose’s improved battlesight zero. The takeaway for me, at the time, was that a 50 yard zero was a good all around setting for backup iron sights on the AR-15 platform. Kyle Lamb said the same in his excellent book, Green Eyes Black Rifles. I did not have a carry handle sight to worry about at the time, as I was sure I was always going use optics with a flip up BUIS, so I never paid attention to the procedure.
As I’ve gotten more interested in iron sight usage, and am now dealing with the shortcomings of the sight settings, I figured it was time to revisit the concept. There is a fairly old, but still fantastic, post by Molon over at AR15.com about taking the Santose IBZ one step further to achieve the 100 yard setting instead of a 50 yard setting (he also added it to the zeroes and trajectories post I linked to on M4carbine). Essentially, I had to loosen the rear sight drum to allow it to click six more notches below the 6/3 setting that is typically considered the “bottom.” For an 8/3 rear sight (found on the A2), it would only be 3 notches due to the coarser increments. Molon recommends adding one extra MOA of “buffer” space to prevent the rear sight housing from making contact with frame (one click on 8/3 sights, two extra clicks on 6/3 sights). This allows for a more repeatable zero. I am using his photos for this, so please check out the linked thread for more info.
Insert a 1/16 allen key and loosen the screw. You don’t have to remove it, just loosen it enough so that the bottom of the sight drum can rotate individually from the numbered portion.
While keeping the allen key and the top half of the drum in place, rotate the bottom half clockwise for eight clicks (only go four if this is for an A2 sight with an 8/3 setting, since each click is a full MOA and not 1/2 MOA). Then tighten the set screw.
You should now be able to turn the rear drum eight more clicks below the official bottom setting. Turning two notches back up (so it’s 6/3 minus 6) will be the new 100 meter setting. Turning two more clicks up (6/3 minus 4) is 50 meters, and the 25m zero remains at the 6/3 marking.
This, supposedly, now allows you to have a 100 meter and 50/200(ish) meter settings while still maintaining the markings on the drum for accuracy at 300, 400, 500, and 600 meters. However, this does require resetting the front sight post to the correct height, since the entire rear sight assembly has been raised by several MOA.
This is all theoretical, of course, and I will confirm it at my next range trip.
Note: The above description of zeroing at 25 meters using the 6/3 sight assumes a carbine length sight radius and barrel. If using the RIBZ on a full size rifle, as I do, then click two notches up to the “Z” setting for your 25 meter zero, and then continue using the 300, 50/200, and 100 meter settings as normal.