Search for orange colored weapons. Purchasing them or receiving them as quest rewards will not count. They must be found in crates or looted from enemies you kill. Consider using Purna as acharacter, as she has a skill with three levels in her survival tree that allow a better chance to find better loot in increments of 5%, 10%, 15% respectively. Drive around the Resort in Act I and search as many named locations as possible (bungalows, gas stations, and even the big resort). Note: Chest rarity does not seem to affect on item rarity.