That 2 MOA is an average for military M-16 A2 with military ammo. Also keep in mind I believe that stat is average group size shot. You have regular grunts behind the trigger for that I believe. Besides there are a lot of civilian AR’s that will outperform military rifles especially accurized ones, H-BARs, and target models.
In regards to M855 ball ammo "Typical accuracy of average lots in an M16A2 is about 2+ MOA"
from:
http://www.ammo-oracle.com/body.htm