The M-16 is a weapon that must be cleaned, very often!
During my six years of service in the Air Force I found the M-16 and its shorter variant,What we in the K-9 field called the GAU to be very good rifles.
The problems were not the rifles the problems were the cheap Ammunition the government procured during the Vietnam war and the period just after that.
If they bought smokeless powder for that Ammo,The M16 would have been a dream.
With the Cheap Ammo that rifle would gum up and become a filthy mess.So it was really important to clean right after use.