Maybe I'm slow as shit with this stuff but I just realized that the new AR isn't as new as I thought it was. It's exactly the same rifle as before but the protective metal casing and the handguard were removed.
http://i48.tinypic.com/2vsozf9.jpg
You can even see where the handguard/grip and the protective casing were attached to the frame of the rifle. The electronics of the rifle like the bullet counter and the compass are exposed due to the missing protection and overall the rifle looks more raw and bare bones that way.
Maybe that's the way these rifles are treated on Reach or it hints at weapon customization and we can consider all the weapons shown so far the bare bones version of their kind. That would also explain the blurred out HUD element.