I’ll explain how to set up UEFN devices to grant weapons and items at random by rarity. With this system, players can get the weapon they chose, but with a random rarity that’s individualized for each player. If the player doesn’t choose a weapon, they’re given a random weapon with a random rarity. Rotation/mobility items and healing items will also be granted at random.
https://dev.epicgames.com/community/learning/tutorials/19b9l/fortnite-rarity-random-items-tutorial