The M16A1 (used in Vietnam) was made out of cast iron , therefore it was namend "The Matel Toy" from the Soldiers!
The main different is: The M16A2 has only single- and 3 burst shot
The M16A1 was susceptible for jam on the gun and used single and automatic fire! that was changed to save Ammo!
The M16A2 was modified so it isn't jam susceptible anymore and it was change to single an three burst shot , the barrel is heavier and has a muzzlebreak and an shelldeflector. The Magazine was changed too , to use the Nato 5.56x45mm (.223) Ammo.
M203 40mm Grenadelauncher: You can demount the front sides of the M16A2 and there you can mount the M203 GL
You see , it is one and the same M16 , you can only mount and GL on every M16A2
Hope thats enough
If you find    spelling mistake , keep them
(i am german i have bad english
)