Yes, even the LMG version of the M-16 had limited success in Vietnam. Not until the M-249 came around about a decade and a half later did they get a reasonably lightweight platform that functioned well and was controlable, albeit belt fed.
The biggest problem with most of the many cartridge variants that M1 carbines had been converted to over the years, was finding one that actually provided a ballistic advantage over the .30 carbine round while confined to the Carbines 40,000 cup max operating pressure. Very few cartridges performed better within this limitation when compared to the M1 Carbines original performance requirements.