From a Reputable dealer such as Colt, Stag,RRA,CMMG,Bushy

Bushmaster-very little but anyone could feel it. RRA feels like one solid peace.

All the time, it is normal and not a bad thing.  Loose is reliable.  It shouldn't be sloppy loose but it should not require tools to take apart either.  One is just as wrong as the other.  

It also has ZERO to do with the accuracy of the weapon.  Much like a 1911 where it is not slide to frame fit that matters, it is slide to barrel and barrel bushing.  Same concept.  

  My first AR(Colt) has wobble between the upper and lower and if you hold it up you can see light between the two (it even came with an Accu Wedge form the factory). It shoots fine though.
  I have seen wobble from just avout all AR manufacturers that I have encountered (I work part time at a gun store). From the six mfg. that we carry it seems that RRA and Armalite have the least wobble. Others may have diffrent experiences, but this is what I have observed.