The assault rifle is inefficient at killing flood combat forms. Infection flood you can just melee like has been said before, the rest just pop when they hit your shield. With a pistol you can take out combat forms from a distance by hitting them in the "head" region. When they start to swarm you, np, because you already thinned them out from a distance with a powerful gun and then you have your shotgun for close combat. AR does not put out enough damage on flood to warrant the amount of ammo used.