Something odd happened while soloing the lamps with my hunter. When I got to the last lamp, I killed a group of thrall with a shot from Fatebringer. A heavy dropped for every one of them. Then I jumped on the rock and every thrall I killed dropped heavy. It was almost like having the ruin wings on.
I *do* have a theory that precision kills are a modifier in the various drop rolls. It's just anecdotal, but I swear I get more heavy ammo and engrams (of any color) when i get precision kills than body shots/AOE kills.