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Enter the M16A1, quite a lot of changes were done to it, and the end result was a very dependable and reliable rifle.
First and foremost, the recoil buffer is made heavier, what this does is it slows down the cyclic rate resulting from the overpressure ammo, it's now normalized and timing problems are eliminated, no longer will the hammer follow the bolt into battery, no longer will the action try to extract too early, no more will the action outrun the magazine, and premature wear is a thing of the past.
Another change is that the chamber and bore of the barrel is plated with chrome, this was an excellent improvement, as not only did it protect the barrel from corrosion, it extended the lifespan of the barrel, made extraction under harsh conditions much more reliable, and it made the barrel very easy to clean.
That's another thing, by the way, though the M16 had been touted by the Army as being self-cleaning, and no cleaning kits thus being issued, this was realized as a terrible error, and thus the M16A1 has a compartment in the stock where you would find an included cleaning kit.
There's this idea that the M16 had this problem of constantly needing to be cleaned, but in reality this is not true, the lack of being able to clean your rifle in the field is obviously a bad thing, but you can actually ignore cleaning the AR15 for tens of thousands of rounds.
What would likely be an issue over time would be the dirtier nature of the new gunpowder combined with the high humidity of Vietnam causing corrosion in the chamber and thus risk possibly very difficult extraction problems (sometimes referred to as 'cartridge swelling' in some sources).
With a protected barrel and a cleaning kit, this was no longer a problem, and with a normalized cyclic rate, it would be extremely unlikely get horrible problems like case head separation.
