Hallucinogenic darts fire silently from the twins' gauntlets and drive the target into a frenzy. Shoot one into a fire or other heat source and it bursts, spreading the toxin over everyone nearby. Bell builds the launcher after the twins intercept a recipe for the serum in Cable News.
One dart takes a single guard out of the picture and turns him against his friends. For groups, look for a fire or brazier near them and shoot the dart into it to gas the whole area.
The dart gun works much like a silent firearm, so it is a good pick when a revolver shot would bring the whole stronghold down on you.