Flare gun in fortnite - In Fortnite Chapter 3 Season 1, you can find a flare gun as random loot, but you can't find extra ammo for it anywhere, so use it sparingly.
The flare gun does two main things: it reveals opponents' locations, and it starts shooting (in particular, it lights up opponents' structures). It also damages other players if you hit them with it, and can even be used to light campfires, but there are easier ways to do both.
If you want to use the flare gun to reveal opponents' locations, then it's best to fire it into the air over the area where you think your opponents are (or aren't) hiding. This is because if the flare hits an object before it detonates, then it won't reveal anyone's location..
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If and when a flare explodes in the air, you'll see a blue ripple effect pass over the environment below it, and then you might see one or two small orange diamonds. These diamonds show you where your opponents are. If you don't see any, then there were no opponents in the flare's area of effect at the time of detonation. Orange diamonds don't last long, so use that recon information wisely.
After launching a flare, a small orange diamond appears in the water, just to the right of that cabin. The other main function of the flare gun is to set things on fire. It will set almost anything made of wood on fire, so it's best not to use it if you're inside a wooden structure of any kind.
The best things to set on fire with the Flare Gun are structures built by other players, especially if they are hiding in or behind them. When you have a flare gun, those players that start building structures around them as soon as you engage them in combat are a lot less annoying and a lot easier to deal with.