There is more to why the XM16E1 (M16) malfunctioned consistently in combat operations in Vietnam. Just as a reference, Vietnam basically consists of a wet and dry season and it either rains constantly or it’s very dry and dusty. And to be clear, the issues being discussed primarily relates to the M16 and not the M16A1
The XM16E1 was issued in 1966 to units in country, to include Special Forces. They were delivered with no training, minimal and often no tool/cleaning kits or lubricating powder solvent, couple that with no weapon maintenance training. That was a disaster waiting to happen from the start before we even mention the other issues.
Due to the rapid escalation in Vietnam, there was a lack of ammunition, which led to a change of the powder specs for the 5.56 ammo. The change was made without carefully examining the modified ammo while shooting under extreme operating conditions. This ammo increased the cyclic rate of fire, thus increasing the use and wear on parts, which resulted in fouling, blockage and jams.
The M14 had a room chromium-striped barrel designed to resist corrosion in combat operations (a lesson learned during WWII). However, the M16 did not have any chromium-striped boring or room. Couple that with the climate of Vietnam, and you now have severe barrel issues, to include rust.
There have been several documented accounts of our troops being killed by the enemy while their rifles were broke down to clean the shit out of them because they wouldn’t function. I used to have pics but got rid of them. Because of all of the above mentioned, a congressional research committee was established. In addition, there were reports/concerns about of the stop pled of the 5.56 ball round and unsatisfactory penetration. This was in comparison to the enemies Russian 7.62x39 round while under fire with light cover.
The outcome was the XM16E1 was modified in early 1967 and the M16A1 was born. The revised rifle was given a boring and room chromium-striped to eliminate corrosion and stuck cartridges, and the mechanism of the boring and retreat of the rifle was reorganized to adapt Army-A 5.56 ammo. Tools and cleaning kits were provided as well as lubricating powder solvents. Intensive training schemes in the cleaning of weapons were instituted, and a M16A1 handbook (comic book) was distributed to the soldiers to demonstrate suitable maintenance.
The problems of reliability decreased rapidly but the reputation of rifle continued to suffer. Complaints about the penetration and unsatisfactory stopping power of the 5.56 persisted throughout the entire conflict/war. The M16s reputation will forever be tarnished because many soldiers/marines died as a direct result of the issues mentioned, which could have been prevented. Although the M16A1 was much more reliable, it still malfunctioned if not properly maintained, whereas the AK almost never malfunctioned, even when not regularly maintained under these extreme conditions.
IMO the AK was the better-suited weapon for the environment and climate in the jungles of Southeast Asia. Most enemy engagements were close range encounters.
Edit: Added pic.