Place more orders, rank 2 you get a class specific legendary, rank 3 you get a class specific exotic.
English
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You also get more options when it comes to your order. By this I mean say its the new arms day (Wednesday) and you have an order to turn in from the previous week for a weapon. When you go to the gun smith you will have more options for the gun based on your reputation. The gun will be the same model with basically identical stats but they will have different weapon perks and element damage if it applies to the gun (secondary and heavy weapons).
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What he said ^