From 21st Century Press Release:

The M16 Multiple Gun Motor Carriage was a variant of the M3 halftrack that was fitted with an armored four gun turret in the rear fighting compartment. The M45 Maxson quad .50 mount was originally developed as an anti-aircraft weapon, but as the war ground to a close and air superiority kept the sky increasingly clear of axis fighter-bombers, the quad mount became more and more used in a ground fighting role. The blistering firepower (2,000 rounds per minute) of the four .50 caliber Browning M2 machine guns was used with devastating effect against soft-skinned vehicles and personnel. The M3 Halftrack was a reliable and highly effective halftrack, and continued to serve into the 1950s, again seeing action in the Korean War.