 FACT: In the climactic scene, Archie is driving the tank while Tommy  is in the Commanders Position (manning the .50 cal). A tank is not  (and cannot be) fired from the drivers position. In addition, we  later see Tommy loading the main round, this is done from the  loaders position which is not easily accessible from within the  tank itself. In addition, Tommy wouldnt be able to fire the round,  should he have loaded the round from the loaders position and made  it back to the commanders position, as the gunner (a fourth man on  a MBT) actually fires the round.