When Dynaco (?) got the plans to the original M16, they decided to make a few minor improvements. The further they got into it, the more improvements they found were necessary; eventually they made more than 150 changes. (I got this from a lengthy article on the internet but no longer recall what it was called. It has been a long time since I read it and I do not remember most of the details -- I think the first idea was to get rid of the carry handle and make our guns among the first flattops -- but the 150+ changes is accurate.)
As a friend who worked there once told me, if you shake an M16 you can hear the parts rattle around inside. Shake a C7 and all you hear is silence.
Keeping up with parts would be a b^tch though - "Hi, ATR, I need another dozen chrome lined AR barrels, four dozen bolts and a dozen carriers" - lovely.
