I'd recommend Jade Rabbit, its probably the gun I use most in PvP. Granted its not ideal for all maps but if you get some space between you and your opponents then you'll be surprised how effective it can be. The trick is to aim for bodyshots, chain a few together and your precision shots can be enough to take out a fresh guardian in a single headshot. But regardless of the scout rifle used remember to play like you're using a scout rifle/DMR and hold back, follow teammates and cover them (i.e. use them as bait) and don't get drawn into close quarters combat.