My uncle was there from 65-66. From some of his stories the M16 was just starting to be properly refined in 65 and he had mixed feelings about it when he as out in the field (canopy). I think he adjusted to it by the time he rotated out in 66.
Watched a video on the Tet fiasco in 1968 as it occurred in Saigon and it showed one part briefly of an ARVN in a firefight in one of the districts and his barrel was literally glowing red from the bayonet lug forward. This was filmed in broad daylight and you could see that barrel glow cherry red in the high-noon daylight. Either he had a thinner pencil barrel or was just simply firing a lot of rounds, or both, before the camera came on.