On 30 March 1962, the first batch of 32 M113s arrived in Vietnam, and were sent to two Army of the Republic of Vietnam (ARVN) mechanized rifle companies, each equipped with 15 of the APCs. On 11 June 1962, the two mechanized units were fielded for the first time. During the Battle of Ap Bac in January 1963, at least fourteen of the exposed .50 caliber gunners aboard the M113s were killed in action, necessitating modifications to improve crew survivability. Makeshift shields formed from metal salvaged from the hulls of sunken ships were soon fitted to the carriers, affording better protection. However it was found that this material could be penetrated by small arms fire, so subsequent shields were constructed from scrapped armored vehicles.