2h gun(rifle) has more base damage than dual 1h guns, however, this also depends on how many advanced classes you have unlocked. Gunslinger should be the first class you unlock if you are a gun user. While you are a gunslinger, all of your skills are usable with both a rifle and dual 1h guns. After mastering gunslinger is when you should make your choice. I have mastered all gun classes and after trying everything, I recommend using dual 1h guns and unlocking double barrel. In my opinion, it is one of the best advanced classes in the game at this point in time. Its quickness combined with the dps(damage per second) output is perfect for both PvP(player vs player) and PvE(player vs environment). Yes, you will have to farm 3 evolution gangues from the advent dungeon, but I promise unlocking this class will be worth it. If advent isn't your thing, you also have the option of unlocking the class with 7 gems. Best of luck fellow rangers.