Not to be confused with the Mechanicum's Flechette Blaster.
Flechette Dischargers are defensive weapons used by the T'au Empire as a reactive, single-shot weapon. It is a support system that consists of powerful clusters of reactive charges attached to the prow of the vehicle. If an enemy moves into close proximity of the vehicle, the dischargers automatically fire off clouds of high velocity flechettes which often prove to be lethal against the would-be attacker. XV107 R'varna Battlesuits are also equipped with Flechette Dischargers.
Alternatively, if the particular Pilot feels that being unable to fire these Flechettes at will is a waste, they could try out the Flechette Dispersal Pods. These are a variant of the vehicle mounted Flechette Discharger, and are equipped on XV109 Y'vahra Battlesuits. Flechette Dispersal Pods differ in that they can be triggered to fire at nearby enemies by the Battlesuit pilot, even if they are not actively assaulting the XV109.
Tl;dr, they are Tau APS (Active Protection Systems).
Why is it unsurprising that the only Shotgun that the Tau has, just so happens to be mounted only on vehicles...