Have recently installed new 16 inch DPMS M4 barrel into old Busy upper with fixed carry handle.  Went to range to sight it in at 50 yards was able to get it within 6' orange pumpkin target but was drifting all over the 6' radius.  While trying to figure out why I was drifting so much noticed that the A2 Rear sight had some play in it.  Is this normal?

In my experience, all A2 sight assemblies have some play in them due to the spring loading in them.  When not being physically manipulated by the user, the sight stays where it is supposed to and returns to that point and does not affect sighting in or point of aim.

If you do not like the slight "swivelling" you can change the rear sight set up to a National Match-type set up that will change the way the rear sight is mounted.  There are various set ups for these types of sights.

Cool, thanks for the info cmdr!

The sight should move from side to side but should return to its original position if you pull it one way and let it go.