The M14 was the weapon most recruits became familiar with in bootcamp, and was originally used first in Vietnam. However, the wooden stock would absorb the moisture from the humid environment in Vietnam, and cause the weapon to weigh more. It fired stronger rounds than the M16, the 7.62mm rounds, being one reason why it was still carried.
The M14 was the descendent of the legendary M1 Garand rifle. Its use was short-lived as a standard issue rifle, because the M16 would become standard for many soldiers, however its stronger rounds were often a reason why it was still carried, and that it was oftentimes more reliable than the M16.
Arques, Antonio. "M14 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,