I think we've gone through this before. I'm dredging from memory of an SOF article some 5 or so years back.
What we now know as the M-16 was designed for ammo loaded with IMR powder. Olin Corp. lobbied the Pentagon for an ammo contract. They used ball powder. This raised the cyclic rate from around 900 or so, up to around 1,100. Ball powder also burned dirtier than IMR.
Factor in Walt's comment about maintenance--particularly in VietNam's climate, and you had a recipe for disaster: More heat from the higher cyclic-rate, powder residue, and moisture...
We know there was a problem. Dead GIs were found, shot while trying to use a cleaning rod to clear a jammed weapon.
The quick-fix (so to speak) solution was the forward-assist.
Now, I won't argue with those who claim that the tolerances and fit of an M16 are just too tight to be all that reliable in sloppy, battlefield environmentsl, particularly after prolonged firefights. Wasn't there, didn't do that.
FWIW, Art