Highly adaptable vehicles running on tank-like tracks, the armored personnel carriers, or APCs, proved to be the backbone of armored cavalry formations during the Vietnam War. The M113 was the most important APC, and it could be modified for use as a carrier for mortars, machine guns, flamethrowers, troops, and command posts. When suitably armed and protected with heavy machine guns and hatch, they could also be used as assault vehicles. The M113 was lightly armored with aluminum and equipped with a .50 caliber Browning heavy machine gun on its roof. Besides its driver, the M113 APC carried eleven infantry troops and a machine gunner. It had a speed of 40 mph on land and almost 4 mph in water. There were more than 2,100 M113 APCs in Vietnam by January 1968. By then, the APC had even evolved into an ACAV, armored cavalry assault vehicle. Armored cavalry units were reequipped with M113s upon arrival in Vietnam, and they modified the vehicle by building armored shields around the .50 caliber machine gun and adding two 7.62mm M60 machine guns.