In Fortnite you can't actually repair weapons, it might be added later though for now here's a suggestion of what to do when items break down. Monitor your weapons or items as you use them until there's just a small bit of durability left. Once it hits that point of small remaining durability, recycle it. That way you get some materials out of it and that goes towards the next construction. You don't lose any value based on how much durability is left on an item so it's worthwhile to get the most out of the weapons before they break. It seems that the game is focused on having players constantly move through weapons and that's an interesting way of going about it. With that, aside from collecting new guns you'll also have to make them.
The schematics for making your own are given through the Loot Llamas and then you'll have to scavenge the right supplies to create weapons. It definitely sucks that you can't hold onto your favorites for long, but it should keep things fresh as you'll constantly burn through what's available. That aside, there are just a ton of guns available with multiple levels of upgrading possible for so much depth there. I hope this was helpful or at least gave somewhat of a tip for what you should do with breaking weapons in Fortnite and good luck surviving in the apocalypse!
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