Rapid Fire used to allow an instant activation of Gauss Barrel (otherwise it's like 3 seconds) without consuming a charge. You could pop Rapid Fire then pop Gauss Barrel immediately to spice up your burst damage, then once you've unloaded all your opening burst, turn off Gauss Barrel and save it for when Rapid Fire is up again, as apparently Gauss Barrel, like Wanderer's Minuet, resulted in an overall DPS decrease due to loss of auto-attack.

Now you cannot do that.