Final answer:
The Marine Corps adopted the M16 rifle in 1967, a year after the U.S. Army, signifying a change in combat tactics during the Vietnam War.
Step-by-step explanation:
The United States Marine Corps adopted the M16 rifle in 1967, following its introduction to the U.S. Army in 1966 for service in the Vietnam War. While the original M16 faced criticism and was involved in a reevaluation of combat tactics due to the changing nature of warfare, it eventually became the standard service rifle for the U.S. military during the Vietnam War and beyond. The adoption of new weapons like the M16 represented a shift in the approach to modern combat, acknowledging the increased importance of rapid firepower in the face of guerrilla tactics and well-entrenched enemies.