From Wikipedia "
The M113 is a fully tracked armored personnel carrier that has formed the backbone of the U.S. Army's mechanized infantry units from the time of its first fielding in Vietnam in April 1962. The M113 was the most widely used armored vehicle of the U.S. Army in the Vietnam War, earning the nickname 'Green Dragon' by some people, but largely known as an APC and ACAV (armored cavalry assault vehicle) by the allied forces, as it was used to break through heavy thickets in the midst of the jungle to attack and overrun enemy positions.
The M113 introduced new aluminum armor that made the vehicle much lighter than earlier vehicles. Thick enough to protect the crew and passengers against small arms fire but light enough that the vehicle was air transportable and moderately amphibious. In the United States Army the M113 series have long been replaced as front-line combat vehicles by the M2 and M3 Bradley but large numbers are still used in support roles such as armored ambulance, mortar carrier, engineer vehicle, command vehicle, etc. The Army's Heavy Brigade Combat Teams are currently equipped with around 6,000 M113s and 4,000 Bradleys. The M113's versatility spawned a wide variety of adaptations that live on worldwide, and in U.S. service. These variants together represent about half of U.S. Army armored vehicles today. To date, it is estimated that over 80,000 M113s of all types have been produced and used by over 50 countries worldwide, making it one of the most widely used armored fighting vehicles of all time. The Military Channel's "Top Ten" series named the M113 the most significant infantry fighting vehicle in history. The U.S. Army plans to retire the M113 family of vehicles by 2018 and is seeking replacement with the GCV Infantry Fighting Vehicle program.
The Vietnam War was the first combat opportunity for "mechanized" infantry, a technically new type of infantry with its roots in the armored infantry of World War II, now using the M113 Armored Personnel Carrier. In addition, Armored Cavalry squadrons in Vietnam consisted largely of M113s, after replacing the intended M114 in a variety of roles, and Armor battalions contained M113s within their headquarters companies, such as the maintenance section, medical section, vehicle recovery section, mortar section, and the scout (reconnaissance) section. U.S. Army mechanized infantry units in Vietnam were fully equipped with the M113 APC/ACAV, which consisted of one headquarters company and three line companies, normally with an authorized strength of approximately 900 men. Ten U.S. mechanized infantry battalions and one mechanized brigade were deployed to Vietnam from 1965 until their departure in 1972: 2/2nd Mechanized Infantry, 1/5th Mechanized Infantry, 2/8th Mechanized Infantry, 1/16th Mechanized Infantry, 2/22nd Mechanized Infantry, 4/23rd Mechanized Infantry, 2/47th Mechanized Infantry, 1/50th Mechanized Infantry, 5/60th Mechanized Infantry, 1/61st Mechanized Infantry, and the 1st Brigade, 5th Infantry Division (Mechanized)."
The U.S. Army 1st Brigade, 5th (Mech) Infantry Division in Vietnam was not composed of strictly mechanized infantry battalions. The 5th (M) ID (1st Bde), consisted of: the 5/4th Field Artillery, 1/11th Light Infantry (straight leg-no armored vehicles), 1/77th Armor (M48 Patton tanks), 1/61st Mechanized Infantry, "A" Troop" 4/12th Armored Cavalry (only one Troop of Cavalry), and the 3/5th Armored Cavalry OPCON (Operationally Controlled) /Attached from the 9th Infantry Division. The one troop of the 12th Armored Cavalry and the full squadron of the 5th Armored Cavalry were M551 Sheridan and M113 ACAV equipped
M113s were instrumental in conducting RIFs (Reconnaissance In Force), Search and Destroy missions, and large invasions (incursions) such as during the U.S. invasion of Cambodia on 1 May 1970 under President Richard Nixon, and later Laos (Operation Lam Son 719) in 1971; all of which utilized the M113 as the primary work horse for moving the ground armies. While operating with US Cavalry and Armor units, the M113s often worked in conjunction with US M48 Patton and M551 Sheridan tanks. During the Vietnam War, U.S. Army gun trucks (modified 2½-ton and 5-ton cargo trucks), along with V-100 armored cars, conducted convoy escorts for military traffic.
The USAF used M113 and M113A1 ACAV vehicles in USAF Security Police Squadrons, which provided air base ground defense support in Vietnam. M113s were supplied to the South Vietnam ARVN forces. One notable ARVN unit equipped with the M113 APC, the 3d Armored Cavalry Squadron, earned the Presidential Unit Citation. M113s were also supplied to the Cambodian government army, equipped with a turret for the machine gun and a recoilless rifle mounted on the roof.
Australian forces used the M113 in Vietnam. After initial experience showed the crew commander was too vulnerable to fire, the Australians tried a number of different guns shields and turrets, eventually standardizing with the Cadillac-Cage T-50 turret fitted with two .30 cal Browning machine guns, or a single .30/single .50 combination. Other turrets were tried as were various gun shields, the main design of which was similar to the gun shield used on the U.S. M113 ACAV version.
In addition, Australians operated an M113 variant fitted with a Saladin armored car turret, with a 76 mm gun as a fire support vehicle, or FSV, for infantry fire support. This has now also been removed from service.
Subsequent to Vietnam all Australian M113 troop carriers were fitted with the T50 turret. The FSV was eventually phased out and replaced with a modernized version known as the MRV (medium reconnaissance vehicle). The MRV featured a Scorpion turret with 76 mm gun, improved fire control, and passive night vision equipment.