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.

The A2 rear sight is spring loaded to one side.  If properly fitted, it looks slightly crooked when all the way down.  This is intentional as the spring loaded feature causes the sight to return to the same plaace every time.

Yes, it rotates, but always returns to zero.

Forgot to say, WELCOME !

Thank you, I have been hearing this from several different places since the post.  I don't know how I never noticed this before.  Anyway, thanks.