It's also standard military hardware for frontline troops. MC uses it because he's initially a standard frontline kind of SPARTAN. His combat skills are great and he's the best jack of all trades fighter, but canonically there are SPARTANs more skilled in various fields than him. His main attribute is his leadership skills and ability to think outside the box.
Also, the AR is a lot better story wise than gameplay. Think of it more like a standard military AR, with the BR being an urban CQC precision weapon and the DMR is a long barrelled shun designed for long range engagements.
Storywise the AR has a range much greater than the 10m it has in gameplay.
Fire Bucket http://img.photobucket.com/albums/v413/N_o_R/ron.gif http://img.photobucket.com/albums/v413/N_o_R/BtJ6F.gif