AR15 sighting problem

[greenhorng]

In sighting in my AR15 using improved battlesight zero, I'm experiencing a problem. Using the small aperture, I get a good zero at 50 yards, although my rear peep is moved pretty far to the left. The problem occurs when I flip to the 0-200 aperture and dial the sight up to 6/3. I get a good grouping but my group is 2-2 1/2 inches left of the small aperture group. I make sure my check weld is consistent and the rear sight is tight. Any suggestions?

 

[henschman]

If this happens consistently and can't be attributed to shooter error, the most likely thing is that the apertures are off center from one another. You might just buy another aperture and see how it works. They are cheap. There is a little bit of windage adjustment that occurs when flipping because the aperture is on a screw, but it shouldn't be any more than 1.5 MOA. certainly not 4-5 MOA like you are getting.

 

[helotaxi]

More likely that the front sight base is canted with relation to the upper receiver. That explains both the need to push the rear sight way off center to get an initial zero and the horizontal shift in POI when you make an elevation adjustment on the rear sight. The canted FSB is basically creating a 2-dimensional problem where a 1-dimensional solution exists.

Match shooters have a fix for this in the form of a windage adjustable front sight base, but that isn't exactly practical for a recreational shooter or a hard use rifle since it involves machining flats on the barrel to allow the FSB to be shifted around on the barrel and deletes the pins that hold the FSB on in favor of a couple of set screws.

 

[henschman]

I don't think it is from a canted front sight... what he is doing shouldn't even result in an elevation adjustment. The large aperture is 2 MOA lower than the small one, but he is adjusting the elevation drum up 2 MOA to compensate. Doing this is supposed to give you the same 50/200m zero, just with the larger aperture.

If you really scrutinize the rear sight apertures, can you tell whether they are drilled off center from one another? That big of a difference might be able to be noticed with the naked eye.