You need to be too close before it is useful, by which time you are probably being mobbed. Fusion rifle is more useful as a heavy hitting back up weapon. Can regularly take out two or three Thralls at close range. Can a shotgun do that? (Honest question I've never upgraded one so don't know!) Of course being in the same slot as the sniper rifle doesn't help either. I'll play around with them once they upgrade the damage. An arc shotgun for taking out knights would be good in nightfall.