The M16A1 was first produced for the Vietnam War. Towards the middle of the war, it replaced the M14 as the U.S. military's MCR and saw extensive combat from then on. However, the rifle had it's fair share of problems.
Throughout the war the M16A1 showed to be an extremely unreliable weapon, prone to jamming, breaking down, exploding, and shredding. Some of this had to do with poor training, and a misguided belief that the weapons was self cleaning and did not require a lot of maintenance. Proper training increased reliability significantly, but U.S. Soldiers and Marines still found themselves having to take extremely good care of their rifles with constant cleaning, disassembling, and oiling.
However, besides the aforementioned unreliability issues, it still proved to be a potent weapon. Able to fire in semi-automatic, burst, and fully automatic firing modes, it was a much more versatile weapon than it's predecessor the M14. It was also lighter and more accurate than the Viet-Kong's AK-47 arsenal. Additionally, the 5.56 ammunition used by the M16 was lighter than the 7.62 ammo previously used in the M14, allowing soldiers to carry more into the field.