When playing in the Shattered Throne Dubgeon, Heavy ammo finder would not drop heavy. I have been in the Vorgeth fight for the last hour and I have only gotten 1 or 2 bricks of heavy.
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It may not be the perk that is broken. But how bungie designed that Dungeon. Cause my fire team did not get many Heavy drops either. Now it is possible that the perk is broken in the dungeon and Bungie is not aware. Or Bungie designed it to be difficult and not have increased heavy drops.