This gun is used to fire flares to create a source of light. Once fired, a flare will hang in the air for a short time and light up the area around it in a red glow. Once it reaches the ground, it extinguishes and the light source goes out immediately.
Flare guns are most commonly found in coastal paddleboat spawns and docks, though can be found in nearly any regular weapon spawn, both civilian and military. Flares can similarly be found in these locations.