Field Scout, hands down. Having a clip of 13 and carrying almost 100 rounds in reserve is the obvious choice. Explosive rounds lower critical damage by around 20%, which is a pretty significant loss, and with Firefly, if you're landing headshots, you don't need Explosive Rounds. They only really come in handy for the slight stagger effect, and for breaking Hive Knight shields with the explosions. But neither of those are enough to warrant losing 1/5th of your precision shot damage. And Single Point Sling is not really worth a perk slot on this gun. I actually never use it on any gun, but with Fatebringer, Field Scout and Explosive Rounds overshadow Single Point Sling immensely.