My heavy ammo barely drops at all, only the other perk works. (More heavy ammo when picked up.)
Edit: Yes, I have the upgrade.
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I find that if I kill a bunch of adds at the exact same time, with striker, grenade, or rocket, it seems they either all drop heavy or most do, not every time obviously but most of the time. Try out splash dmg in tight groups. Works great for me! Think it's a glitch.