“Too much calcium carbonate in the ball powder clogged the gas tube.”
That plus the brilliant minds of Robert McNamara’s Defense Department told the grunts that this was such a great rifle that it NEVER needed cleaning which is absolute nonsense with any mechanical device, especially in an uber-humid environment like Viet Nam.
If you're going to bash McNamara over the M-16, don't forget that he refused to allow Colt to chrome line the barrel and chamber, which would have mitigated some of the fouling issues.