"It's 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.
Shooting from the booster gun has an action cost of 3000, so you normally won't act for two next turns after using it.
The Booster Gun is sometimes obtained by disassembling or disarming a BoosterBot.
The Dart Gun is mostly identical, except it fires at a normal speed.