How many gold patrols do you have to do to just find one?
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If you complete the campaign on all characters and do the from zero and to hero missions you will get 3 red border of all neomuna guns. You can get other 2 by heroic patrols and rank up rewards from nimbus. Also ıf you change the map with your sparrow after terminal overload you can get 4 chests and rank up easier. With that cycle by the time i reach rank 67 i got all the red borders. I hope this helps.