I have experienced my gun fire sound getting delayed on numeral occasions during Private Beta and Open Beta.
Primary it was happening when there is a significant amount of baddies and friendlies on the scene shooting at each other - Control Points for example.
Both Primary and Secondary guns would have the sound missing during the combat. However, the sound kicks back in once the fight is finished - CP captured.
As it appears, the gun fire sound would continue non-stop unless i force stop it by shooting a quick round.