First person to get a good combat accurate hit usually wins the fight, or so the studies say. Studies also elude to fights being in the 3-5 yard range and are over in 3 seconds or so. So I would say a defensive rifle in the 2-3 MOA range and a shooter that is capable of combat accurate hits, with a very good amount of speed.

Combat accuracy is upper center mass if presented. So on a full frontal torso you are looking at about 8"-10" circle centered in the upper chest.

Of course things are a bit different when you hauling ass cause your getting shot at and you are trying to return with accurate fire.