in krav maga and systema, they teach you that realistically against an armed opponent you have 6-7 feet to effectively close the distance, any further out and you're most likely going to be shot and killed, which sounds about right..a trained athletic martial artist can react quick enough to close 6 feet of distance. Bu the time the armed opponent raises, aims, makes sure the round is chambered and safety is off you will most likely be on him...then you have to take into account if you can overpower him for control of the weapon... if you're 5'9 170 and he's 6'3 230...you're fucked either way.