"A small crossbow with a wide barrel to hold darts"
The Booster gun fires Tonics. This can be helpful to debuff dangerous enemies, or to inflict Bleeding by firing empty injectors. It is also useful for healing allies, although healing salves should probably be reserved for the player.
The Booster Gun is sometimes obtained by disassembling or disarming a BoosterBot.
The Dart Gun functions identically.
A Booster gun is, somewhat inexplicably, classified as a Heavy Weapon.