A little research reveals that the patrol boat was not actually part of the North Vietnamese People's Navy. It was actually a junked U.S. Navy cruiser and gunship that was salvaged by the communists after the war, who fixed it up. This is not only historically accurate and likely, but would also explain an American anti-aircraft gun on a supposedly Southeast Asian vessel.
By attacking the soldiers, the rest of the army is distracted and the mercenaries escape and get weapons. Rambo fires the gun empty, and during this time gets shot in the shoulder. Rambo cries out in pain but as the Karen Rebels arrive and he realizes that a truck full of arriving soldiers is approaching. Rambo gets back up, ignores the pain and reloads the machine gun with another belt of .50 BMG links, cocking it and using the machine gun to blast the truck to pieces, slaughtering the soldiers.
Rambo then turns the machine gun on a Tatmadaw river patrol boat, which is using both a flamethrower and a machine gun to fire on the civillians from the shore. Rambo uses the remainder of the bullets to destroy the boat, punching huge holes through the metal walls of the cabin and blowing through the sailor's chests. After Schoolboy helps Rambo by shooting the last machine gunner in the head with a Barrett M82 sniper rifle, Rambo stops firing and sees Major Tint trying to run from his defeat. Rambo jumps off the truck and then kills Tint with his machete. Thus, estimated the entire forces of Tatmadaw for the Karen Rebels victory over the Armies and Missionary safety returning back to United States.
The scene has become quite famous for its level of graphic violence, and is one of the most popular scenes in the entire franchise, and Rambo's last stand is very similar to how WWII war hero Audie Murphy won the Congressional Medal of Honor, manning a vehicle-mounted.50 caliber machine gun and singlehandedly holding off hundreds of enemy soldiers, saving his platoon. Coincidentally, in his commentary for First Blood, author David Morrell cites the primary inspiraction for John Rambo as being Murphy, who would later become a hollywood actor.