M113 A1 ACAV 4/12 CAV Vietnam
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Until Vietnam, the US Army's doctrine had been that infantry units should dismount before assaulting an enemy position. However, as the ARVN discovered, this meant that when facing the massive amounts of firepower that the NLF or VPA could bring to bear during a firefight, the infantry was exposed to needless casualties, as well as losing the momentum of the attack. Indeed it was the ARVN which pioneered the use of mounted tactics from APC's when they first deployed the M113 in 1962. They were also the first to discover the need for increased firepower on the vehicle by mounting an extra .30 Cal. MMG beside the commander, fired by an exposed prone soldier lying on the roof of the vehicle. Perhaps more importantly, they also discovered the vulnerability of the exposed track commander when manning the pintle mounted .50 Cal. HMG during the battle of Ap Bac where 14 out of 17 commanders became casualties.
The US Cavalry units, perhaps smarting under the loss of their beloved tanks, took to the idea and improved upon it by creating the ACAV. They added armour around the commander and a gun shield for the .50 Cal., provided two extra M60 GPMG's each athwart the roof hatch (protected by shields) and installed an M79 Grenadier inside the troop compartment, firing through the roof hatch to provide close support. The result was a vehicle which was able to go where tanks weren't, by virtue of its lighter weight and ground pressure, packed considerable firepower and was agile and reasonably well armoured. The result, when coupled with the aggressive leadership and tactics of the US Cavalry's commanders was highly effective by all accounts.