How do I tighten it up?  It is very, very loose if you adjust it up in elevation.

Thanks

If you are talking about a little rotation from the left to right of the entire rear sight housing, that's normal.

Well, don't know how to explain it other than saying it is loose from side to side and front to back.  None of my rifles are like this (this is a friend's newest acquisition)

First you need to determine where the looseness is. If the aperture itself wobbles left to right and back and forth the only way to tighten it is to selectively replace the aperture, windage screw or both. If the base moves rotation wise it is normal, if it moves to easily the spring needs to be replaced. The spring cams the base to one side and holds it there.

If the whole base moves back and forward especially when its run up, it is either the aggregate play in the elevation wheel / base stem or the fit of the stem in the the upper receiver, most likely the wheel /stem. Both of these conditions would be corrected by replacing either or both.

There are other areas of concern but the above mentioned is most likely where the problem is and is not worth worrying about unless a match sight of dial indicator repatability is desired.