I have noticed that the ammo finder perks on armor only work about 20% of the time, when they do work, I can get a ton of drops but when they don't (they usually don't) I can get maybe 3 or 4 heavy bricks (at the most) with 3 heavy ammo finders on during an entire activity.
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Oh, don't get me started on special/heavy ammo finders. They work in places where you need it the least. In places like Gambit and The Forge, I'm convinced they don't work at all. Of course, the game doesn't tell you this.