ok, I shot two AR's today.  A RRA 20"dcm with iron sights, and a m4 AR15 from bushmaster.  The m4 was equipped with arms 51 mount and a Eotech.  

At 100yards, I was shooting 1" groups with the RRA DCM.  (winchester 45 grain 223)

At 50yards, I was shooting 6" groups with the m4. (LC XM193  from '02).  

Now, I know the DCM has a floated barrel, and the  m4 does not.  The ARMS mount is not floated, but is an extremely rigid setup.

Has anyone every been unsatisfied with the accuracy of a non free floated arms 51 setup?

thx,
MG