There was nothing wrong with the M16, it worked very well for a number of years. Then the US Army unilaterally modified the ammunition to save money in such a way as to make it no longer within specification for the weapon. Predictably, the use of out-of-spec ammunition caused issues.

The Army never changed the ammunition back. Instead, the weapon was modified (M16A1) to be compatible with the formerly out-of-spec ammunition and the issues went away.

You can't blame the M16 for the US Army using ammunition that wasn't fit for purpose.