The M14 was the U.S. Military's main combat rifle (MCR) just after the Korean conflict, serving through the early Vietnam era. The rifle still sees limited use by infantry in the United States Army and United States Marine Corps, primarily in the designated marksman role. The United States Navy still uses a small number for arming shipboard security personnel.
The M14 was originally designed as a sort of squad-level jack of all trades weapon to replace the numerous weapons sported by infantry squads by the end of World War II. The Military wanted to integrate the best features of the M1 Garand, the M3 Grease Gun, the Thompson, the BAR, and the M1 Carbine into a single weapon to simplify training and acquisitions. Weapons designers, including John Garand, subsequently produced what is essentially a direct descendant of the M1, evolving it's basic architecture to suit the new mission rather than starting from a clean sheet of paper. Although the two rifles do not share components, when side-by-side the lineage is easy to see.
As M14 development was hitting its apex, the United States and its NATO allies were wrapping up development of a new standard round. The size of the .30-06 round used by the M1 Garand was poorly suited to automatic weapons, and with automatic fire seen as a key feature of new weaponry a new round was needed. The 7.62x51mm NATO round was designed to have nearly identical performance in a slightly more compact configuration. M14 became the first US weapon fielded in 7.62x51.
Although rugged, lethal, and dependable, M14's time as the Military's MCR was short lived. Unfortunately, real-world experience with the M14 and other 7.62 NATO weapons soon showed that even the more compact round was too much. No infantryman could control a 9-lb .30 caliber weapon on full automatic, and the weight of the weapon and ammunition was higher than desired for a standard-issue. The US Military's next MCR, the M-16, would abandon the large-caliber "Battle Rifle" approach in favor of the compact and lightweight features which are hallmarks of the modern "Assault Rifle." Dedicated light machine guns would take over the automatic fire role, much as they had in WW II.
Although 1970 was the last year M14 was standard-issue; it's lethality, ruggedness, and long-range accuracy ensured it would never be completely phased out. While no longer issued as a front line infantry weapon, some members of U.S. ground forces still choose to field the M14 as a designated marksman's rifle or hybrid sniper because of it's accuracy, dependability and stopping power. After years appearing only in special forces or in the hands of Navy sailors standing watch, M14s have re-appeared throughout the US military during the Iraq and Afghanistan wars. In Iraq, urban structures which can stop or deflect the M-16's smaller 5.56mm round have much less effect on the M14's 7.62mm. In Afghanistan, battles are often fought at ranges beyond the 400-500m max effective range of the M-16 family but still well within the M14's effective range of over 800m (scoped).
M14 has spawned several variants. The M21 and M25 sniper rifles are M14s with match-grade components and no automatic fire feature. The Enhanced Battle Rifle is an M14 with a modular chassis featuring multiple rail mounts, designed specifically for the US Navy SEALs. The M1A is family of semiautomatic-only variants sold to civilians.