Well, by shooting into a wall you can see where the bullets make marks and get an idea of how accurate overall your gun is and how effective different perks are. Yeah, if you play a lot of PvE where accuracy matters a bit less, Field Scout is the way to go for sure. One useful perk on Perun's Fire for PvE is Performance Bonus. So long as you get a kill with each round, your ammo will usually regenerate faster than you can deplete it. Great for slaying waves of mobs with just your Fusion Rifle, especially if you get Armor Piercing Rounds or Grenadier.