I was trying to zero my AR-15 for 50yrds/200m and needed to adjust the bottom out elevation on the rear sight. When I used the Allen wrench to loosen the screw the rear sight became loose. When looking at the rifle from the top the rear sight will rotate a small distance (about 1/16") back and forth. There seems to be some tension pushing the rear sight counter clockwise. When the sight is raised to the highest level it becomes tight and does not rotate, but when lowered back down it happens again. I set the sight back to its original configuration and the problem did not go away. This was the only adjustment I made and I don't see how this has happened. How can I tighten up the rear sight to stop the rotation?

My rifle is a Colt match target competition HBAR, and has the 6/3 with "half clicks" on the rear elevation sight.

Do you mean that the back half wiggles? If so this is normal, nothing you can do about it.

Wait a min, you mean the hood is rotated correct? if so that is also normal, the site has counter pressure from the elev spring

The part that is rotating is the entire rear sight base (the part that contains the rear sight aperture).  I do not remember the sight ever rotating at all since I bought it in 1999.  It feels like there is counter pressure pushing it counter clockwise.

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Correct that is normal, the internal spring is creating the pressure. Most guys just live with it, you can get a very thin piece of clear plastic and insert it between the site tower and the protective hood if you wish.

Thank you.  I don't know how I never noticed it before.  It has been driving me crazy thinking that something was wrong, and that I caused it.  I don't know if I could ever forgive myself if I let something happen to my AR...  Anyway thanks.