Fast rate of fire guns in pvp like praedyth's revenge. For pve high impact ones will do better like ldr for example. Not saying these gun types are locked in that game mode but in pvp, all snipers are a two shot body shot (excluding bonus damage modifiers) so a fast rate of fire will quickly take an enemy by surprise. In pvp the low impact makes damaging larger targets a waste of ammo. You spend like 2-4 with a low impact sniper when a high impact one could have used 1-2