 The F @-@ 4 's biggest weakness , as it was initially designed , was its lack of an internal cannon . For a brief period , doctrine held that turning combat would be impossible at supersonic speeds and little effort was made to teach pilots air combat maneuvering . In reality , engagements quickly became subsonic , as pilots would slow down in an effort to get behind their adversaries . Furthermore , the relatively new heat @-@ seeking and radar @-@ guided missiles at the time were frequently reported as unreliable and pilots had to use multiple shots ( also known as ripple @-@ firing ) , just to hit one enemy fighter . To compound the problem , rules of engagement in Vietnam precluded long @-@ range missile attacks in most instances , as visual identification was normally required . Many pilots found themselves on the tail of an enemy aircraft but too close to fire short @-@ range Falcons or Sidewinders . Although by 1965 USAF F @-@ 4Cs began carrying SUU @-@ 16 external <unk> containing a 20 mm ( .79 in ) M61A1 Vulcan Gatling cannon , USAF cockpits were not equipped with lead @-@ computing gunsights until the introduction of the SUU @-@ 23 , virtually assuring a miss in a maneuvering fight . Some Marine Corps aircraft carried two pods for strafing . In addition to the loss of performance due to drag , combat showed the externally mounted cannon to be inaccurate unless frequently <unk> , yet far more cost @-@ effective than missiles . The lack of a cannon was finally addressed by adding an internally mounted 20 mm ( .79 in ) M61A1 Vulcan on the F @-@ 4E . 
