You can do a Google Image Search for 'M16A2 optics' to see how the optics are mounted to the top of the carrying handle (which I imagine would stink for ergonomic purposes), or see some custom mounts that put the optics in front of the handle. But again, excepting in rare circumstances as mentioned by Eddie, the average grunt in the late 80's had nothing but iron sights.
This is my personal favorite:
http://www.schelin.org/ikonboard/smilies/whatm16.jpg