The flare gun is type of incendiary weapon in Fallout 4.
Flare guns are a single-shot incendiary device that has been repurposed by the Commonwealth Minutemen, in order to call for backup during dire encounters out in the Commonwealth. Alternatively, firing a flare at an enemy for a underwhelming amount of both physical and radiation damage.
In order to use flare guns effectively, they must be fired within a reasonable range of any settlement that is occupied by Minutemen, or Minutemen sympathizers. If backup sees the flare, they will send up their own flare in response to confirm that help is on the way. Soon after, anywhere between three to six generic Minutemen will come to your aid.
Flare guns cannot be modified, for either combat, support, or cosmetic purposes.