The M113 Armored Personnel Carrier (APC) is one of the most-produced post-war armored vehicles in the world. It was originally developed in the 50’s by FMC (Food Machine Corporation) as a more affordable and reliable replacement to the M59 and M75 APCs. One of the major upgrades over its predecessors was the use of military-grade aluminum for the armor that weighed less than steel while offering the same level of protection. In its original form, however, the vehicle was only protected against small arms fire and shell fragments, anything larger (such as the Soviet heavy machineguns) went right through.
The M113 entered the U.S. Army service in 1960. It was a boxy vehicle that could carry eleven men and two crewmembers (a driver and a commander) and it really was just a "battle taxi" designed to drive the American soldiers to the battlefield and back, not to withstand frontline combat. The early version of the M113 was equipped with a 209 horsepower gasoline Chrysler 75M engine, but soon after coming into service (in 1964, four years after its introduction), the vehicle engine was replaced by the 6V-53 215 horsepower diesel and the M113 continued to use diesel engines for the rest of its production (the engine was upgraded further in 1979 by turbocharging it to 275 horsepower). The most common armament consisted of a .50cal Browning M2 machinegun.
The vehicle first saw active combat in Vietnam and, all things considered, it did fairly well. It was capable of performing the role it was designed for, although pressing it into more combat-heavy situations often did not have the desired effect as it was quite vulnerable. By that time, it was also being used by the Australians and supplied to local South-Vietnamese troops.
It is estimated that more than 80 thousand M113 family members (including ambulances, mortar carries and other variants) have been produced since 1960. The M113 was sold all over the world to about 60 countries and organizations with at least 50 still currently using it actively in one form or another. While the U.S. Army itself replaced the M113s with the Bradley Fighting Vehicle family a long time ago, it too still keeps a considerable stock of either actively used or mothballed M113s, some of which were used as late as in War in Iraq – in the first month of the 2003 war, 23 American M113s were destroyed and by the time the war was officially over, 160 of them had to be shipped back to the USA for repairs. Another recent user of the M113 is Israel although there were a number of critical reports regarding the protection offered to the crew and soldiers inside.
There were multiple attempts throughout the M113 lifespan to upgrade its armor as well; the usual solution was to bolt on additional (steel) armor plates. That way it was actually possible to protect the vehicle against up to 14.5mm machinegun rounds. There are multiple kits available for the M113 even today; one of the best known upgrade kits is the Israeli "Zelda". But armor was not the only thing upgraded through the years. There are countless variants of M113 upgrades available and some of them feature various turrets (possibly the best known is the Australian M113 variant with the 76mm Saladin turret).