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Of course, this isn’t easy to do. Players report needing to complete dozens of matches before reaching Rank 16. There are also some reports that claim, once reaching Rank 16, there will be another quest you’ll need to complete to unlock the catalyst, though the specifics at this time are a bit unclear.
That being said, the quests in-game should outline the necessary requirements as you advance closer to earning this gun and catalyst.
Source: https://www.distractify.com/p/how-to-get-forerunner-catalyst-destiny-2
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