An M16A1 has quite a few differences from the M16A2.
There were also several transitional changes during the civilian AR15 evolution.

Here is a link to another thread discussing it
http://smith-wessonforum.com/firearm...#post139626104

Please excuse the missing pics as Photobucket has decided to block 3rd party sharing unless they shake you down for $500...
need to transfer all my gun pics to Imageshack