The Enemy of my Enemy is one of the last main missions in game and won’t be unlocked before you’ve completed all other main missions that require you to establish radio contact with outside world. Those include: Calling for Help, Over and Out, Line of Fire, Radio Silence, Loud and Clear.
Either that, or if you want to get the 5* AI-76 faster, you can trade it with someone.
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And that is the reason why i locked your topic.