The M113 was developed to provide an air-mobile, survivable and reliable light tracked vehicle able to be air-lifted, air-dropped and parachuted by C130, C141, C-5 and C-17. Though initially US Army generals wanted to use it only to transport troops, protected against small-arms fire and shell fragments, to the front line where they would disembark, according to an outdated battle doctrine. The Vietnam War showed that the M113 could be of more use with the crew fighting mounted, which led to the development of the famous ACAV variant by the Army of the Republic of Vietnam (ARVN), a design which was later adapted by the US Army.
Entering service with the U.S. Army in 1960, the M113 requires only two crewmen, a driver and a commander, and carries eleven passengers inside the vehicle. Its main armament is a single .50-caliber (12.7 mm) M2 Browning machine gun operated by the commander.
On 30 March 1962 the first batch of 32 M113's arrived in country, creating two ARVN Mechanized Rifle Companies, each equipped with 15 APCs (M113's).  On 11 June 1962, the two mechanized units were fielded for the first time,  but without the added ACAV sets, which consisted of gun shields and belly armor. During the Battle of Ap Bac in January 1963, at least fourteen .50 caliber gunners aboard the M113's had been killed in action, necessitating M113 modifications to improve crew survivability. The .50-caliber machinegunner's position exposed the gunner to enemy small-arms fire. They soon fitted makeshift shields salvaged from the hull(s) of sunken ships, finding that this material could be penetrated by small arms fire, the following shields were constructed from scrapped armored vehicles.
The ARVN 80th Ordnance Unit in South Vietnam developed the shield idea further and commenced engineering general issue gun shields for the M113 Armored Personnel Carriers.  These shields became the predecessor to the standardized Armored Cavalry Assault Vehicle (or ACAV)...