Perhaps I can explain it a bit better.
Basically swarm tag works with primary turrets as well. You can fire multiple pulses in a salvo with swarm tag. Unfortunately FS2 assumes that whatever weapon the turret is equipped with, has it's own targeting system. Eg, hornet or tornado missiles. Naturally pulses do not track their targets. This makes swarm tagged primary turrrets really inaccurate.
Several attempts have been made to correct this issue but none of them has gained any popularity. I haven't used fs2_open or tbp_open for a really long time, so I don't know whether this has been fixed already or not.