M16A2, that US Army still uses, that im aware of, has semi and burst modes.
There however is M16A3 that i know, but i don't think it exists in large quantities yet, nor produced so.
In the even of such big war as Vietnam was, there wouldn't be enough of these "refined" soldiers.
Garand was useful even after WWII for long time, but I wouldn't call the firepower it gave as 'excellent'
Korean war didn't prove much, as ground forces were mostly what they were and various guns were used, mostly not newest ones.
If it would been prepared army that didn't rely on masses of soldiers and witohut weapons with only importance being 'as long as it kills', it'd been much more even firepower.