Although the M1 was among the most advanced infantry rifle designs of the late 1930s, it was not without faults, as various modifications to the basic M1 were considered during the last months of World War II.

Changes included the addition of fully automatic fire and the replacement of the eight-round en bloc clip with a detachable box magazine holding 20 rounds.

[13] After a series of revisions by Earle Harvey and the other members of the .30 light rifle design group following the 1950 Fort Benning tests, the T25 was renamed the T47.

[13] During 1952–53, testing proved the T48 and the T44 to be roughly comparable in performance, with the T48 holding an advantage in ease of field stripping and dust resistance, and a longer product development lead time.

[13][15][17] FN engineers opened the gas ports in an attempt to improve functioning, but this caused early/violent extraction and broken parts as a result of the increased pressures.

[13] The T44 was selected over the T48/FAL due to lower weight, simplicity with fewer parts, the T44's self-compensating gas system, and because the T44 could supposedly be manufactured on existing machinery built for the M1 rifle (which was later found to be false).

[20] After the M14's adoption, Springfield Armory began tooling a new production line in 1958, delivering the first service rifles to the U.S. Army in July 1959.

Springfield Armory records reflect that M14 manufacture ended as TRW, fulfilling its second contract, delivered its final production increment in the fiscal year 1965 (1 July 1964 – 30 June 1965).

[21] The intention was to simplify the logistical requirements of the troops by limiting the types of ammunition and parts needed to be supplied, but replacing all these weapons proved to be an impossible task; the M14 was deemed "completely inferior" to even the World War II M1 Garand in a September 1962 report by the comptroller for the U.S. Department of Defense.

[23] The rifle was unwieldy in the thick brush of Vietnam due to its length and weight, and the traditional wood stocks made of walnut and birch tended to swell and expand in the heavy moisture of the jungle, adversely affecting accuracy.

Though production of the M14 was officially discontinued, some discontented troops managed to persist with them while deriding the early-model M16 as a frail and underpowered "Mattel toy" that was prone to jamming.

[26][27] A Congressional investigation later discovered these characteristics to be the result of intentional attempts by Army bureaucracy to sabotage the M16's field performance in Vietnam.

[33][34] The 1st Battalion of the 3rd United States Infantry Regiment ("The Old Guard") in the Military District of Washington is the sole remaining regular U.S. Army combat field unit where the M14 is still issued as the standard rifle, along with a chromed bayonet and an extra wooden stock with a white sling for military funerals, parades, and other ceremonies.

[38] According to Black Hawk Down: A Story of Modern War, an account of the Battle of Mogadishu, Sergeant First Class Randy Shughart used an M14 for sniping from helicopters to provide support fire to ground troops.

[5] Original-equipment walnut and birch stocks carry the Department of Defense acceptance stamp or cartouche, consisting of an arc of three stars above a spread-winged eagle.

Although M14 rifle production ended in 1964, the limited standard status of the weapon resulted in the continued manufacture of accessories and spare parts into the late 1960s and beyond.

It featured a heavier barrel and stock, two pistol grips (one fixed, one folding), a hinged butt plate, a selector switch for fully automatic fire, a bipod, and used the standard BAR sling.

Accuracy and control problems with this variant led to the addition of a pistol grip, a folding rubber-covered metal foregrip and a muzzle stabilizer.

It had a full pistol-gripped in-line stock to control recoil, a plastic upper forend to save weight, a muzzle compensator, the BAR sling, an M2 bipod, a folding metal vertical foregrip mounted under the forend of the stock, and a rubber recoil shoulder pad under the hinged butt plate.

Although an improvement over the M14 when in full-auto, it was still difficult to control, overheated rapidly, and the 20-round magazine limited its ability to deliver suppressive fire.

Paragraph 2 stated that the Director of the Alcohol and Tobacco Tax Division, Internal Revenue Service, Department of the Treasury (predecessor to the Bureau of Alcohol, Tobacco, Firearms, and Explosives) had ruled that M14M and M14NM rifles so modified would not be subject to the 1934 National Firearms Act (NFA) and, as such, could be sold or issued to civilians.

Three years later, with the passage of the Gun Control Act of 1968, the NFA was amended to prohibit the sale of previously modified automatic weapons such as the M14M and M14NM to civilians.

The Mk 14 Enhanced Battle Rifle is a more tactical version of the M14, with a shorter 18-inch barrel, a retractable stock and multiple rails for more accessories.

Modified M14 DMR fitted with the same stock as Mk 14, used by the United States Marine Corps; being replaced by the M110 Semi-Automatic Sniper System.

[citation needed] The AWC G2A is a modified M14 with bullpup stock designed by Lynn McWilliams and Gale McMillian in the late 1990s.

Rifles in the 93XX serial range and higher have modified receivers designed to accept Chinese-made bolts, barrels, and other parts owing to a shortage of original USGI components.

[52] Production of these M14s were contracted out to Yunnan Xiyi Industry Company Limited or State Arsenal 356 from the rifles to the 7.62 mm NATO magazines.

[58][59] The U.S. Department of Defense contracted with Smith Enterprise to build and modify M14 rifles for use by soldiers, marines, and sailors in Iraq and Afghanistan.

[58][61][62] The company's history included originally making forged receivers for M14 rifles and briefly switching to investment casting.

The rifle used a medium heavy-weight 18-inch barrel and was used as a basis to create the US Navy's Mark 14 Mod 0, with Springfield Armory, Inc. being asked to supply the necessary machinery in cooperation with the Naval Surface Warfare Center Crane Division.