It is a fixed weapon, found only during the level "Seaside Stowaway, and mounted upon a destroyed jeep. It has unlimited ammunition and has a high rate of fire, making it very similar to the German MG-42. The man mounting the gun will say, as you approach him, "I can't move, take over the gun" before dying.
The M1919A4 sees heavy usage by the United States Marine Corps throughout the Pacific campaign. There is one in each foxhole during the battle of Bloody Ridge, which the Americans use to stall the Japanese attackers from overrunning Henderson airfield. They are also used in standalone emplacements, usually at American checkpoints. There is also one mounted on the LVT during the assault on Tarawa Atoll, which Conlin uses to defend the LVT during its initial ride into the beach.
The land version of the M1919A4 fires from a 100-round drum, with decent accuracy and power. It has a relatively long reload, and usually comes with between 400 and 500 rounds in backup. The LVT version has unlimited ammunition, but retains its reload.