The M16 is easily one of the most recognizable weapons to emerge from the Vietnam War. The earliest model, the M16VN, featured a short mag and teardrop shaped barrel, which would be replaced later in the war by the slightly more reliable M16A1, which kept the barrel and added a larger mag. It fires 5.56mm bullets.
The history of the M16VN, the first model of M16, was given to special operations soldiers in 1962. Upon the beginning of the Vietnam war, the M16 was launched and given to soldiers, one of the first units bringing the weapon into action being the 1st Air Cavalry. The weapon was prone to jamming, despite being described as not needing cleaning and less likely to jam.
The M16A1 was the new and improved model of the M16, the most noticeable new function being a lever, which when used, would push the bolt to hopefully unjam the weapon. It still was not very reliable, but better than the M16VN.
Arques, Antonio. "M16 Assault Rifle." Grunt: A Pictorial Report on the US Infantry's Gear and Life during the Vietnam War 1965-1975, by Arques, Madrid, Andrea Press,
2014, pp. 346-47.