I think a more eloquent solution would be to have HM have a 10% or so chance of dropping an elemental core that you can "infuse" into any legendary or exotic primary weapon thus still affording weapon diversity and also giving people (so far because I cannot see any tangible reason) a reason to grind HM.
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Why not the fusion coils? They barely have a use currently, and the high price is fitting.
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More like 50%
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I mention that very thing in the second paragraph
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My bad. That's what I get for just reading the headlines. Sorry.