Incorrectly regarded as a factual error: M-2 .50-caliber machine guns were routinely used by and fired from USMC Jeeps from WW II until the introduction of the larger Humvee.
A machine gun of the size featured in this series cannot be fired accurately from such a jeep as the Rats'. A U.S. military officer in Vietnam discovered this the hard way when, inspired by the show, he had a heavy machine gun mounted on his own jeep. Several of his soldiers took turns firing the weapon, whose recoil nearly capsized the jeep, and caused rounds to fly in every direction except that of the designated target. (So much for jumping sand dunes and blowing up tanks from the back of a jeep!)
While the machine guns mounted in the rear of the jeeps for the opening sequence do appear to be M2 Browning .50 Cal machine guns, the ones used during the show appear to be cosmetically modified M1919 Browning .30 Cal machine guns in that the barrel is shorter and the muzzle appears to be .30 cal, but the pistol grip was removed and the double hand grip of the M2 style was added.
The shots of the jeeps jumping the sand dune which opens the show (actually the same shot, repeated) are reversed, or "flipped" images. The steering wheel and driver are on the right side of the jeeps in the shots, incorrect for an American vehicle.
The opening credits depict a soldier with a filtered cigarette between his lips. Filtered cigarettes were less than 1 percent of the domestic market five years after World War II ended, making it extremely unlikely that any were shipped to GIs overseas during the war.