Title pretty much says it all. I was running the REAPER: Reaping with the basics, which requires you to "Defeat enemies with weapons that use Primary ammo." However my bow wasn't counting toward the bounty's progress. I was using The Spireful Fang, but I'm assuming all bows aren't counting as primary ammo weapons. When I swapped to my Tigerspite autorifle the bounty progressed as expected.