In the years leading up to the Vietnam war, American fighter aircraft were being built to intercept Russian bombers with missles, hence finding guns on these aircraft was rare. They were sorely missed by the American aviators who found themselves fighting against North Vietnamese pilots who flew nimble, Russian built, gun equpped fighters. Washington's rules of engagement at the time required American pilots to visually identify their targets before shooting. This negated most of the advantage of the missles they carried.
The F8 was intended to be the last American fighter that carried guns and missles. Aircraft of this type shot down 19 Russian built MiG's during the war. Of those shot down, 16 were MiG-17s and 3 were MiG-21s