I ran a full 100 marks worth of Roc strikes on 2 characters last week and didn't get one weapon engram out of those nearly 35 strikes, so either they patched it after Planet Destiny posted the vid a couple months ago, or they're insanely rare.
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They're rare, but you also might be unlucky haha
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None of the people in my strikes got any either, so 100+ rewards and zero engrams makes me think it has been patched, or like I said, insanely rare, i.e. not worth the time.
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I have friends who have gotten one