As far as I can tell, that group doesn't look too bad for a stock DPMS AR.  Depending on ammo, MOA groups are about right.  The easiest thing you can do to tighten up groups is replace the trigger - stock DPMS triggers have about a mile of creep in them.  I also use an accu-wedge in mine, but I cant really say if that affected accuracy much, if any.  If you want little cloverleaf groups out of it, that would involve handloading and tailoring a load to the rifle.