If your attacker is within 21 feet, then he can usually get to you before you can stop him (except with a head shot). If he suddenly rushes towards you, you will have perception time (wow, this guy is actually rushing me, even though I have a gun pointed at him), and reaction time (should I shoot, move, both?). In a well trained person, this total time is about 0.8 seconds. In an untrained person, it is 1.5-2.2 seconds, even after telling the untrained person what the attacker is going to do. In California, you have no DUTY to retreat if attacked, but you may want to move away (usually laterally, not backwards) to give yourself more distance (and therefore time). While you are doing this, I would be giving clear and loud commands. Have him put his hands up all the way, and kneel down. This position allows you the advantage over time. If his hands are empty, it simply takes him too long to get to a weapon (or you) and cannot overcome your perception and reaction time. If he gets into that position, tell him that if he moves or drops his hands, you will assume that he is going for a weapon or is going to attack you, and you will shoot him. If others are nearby, say it loud enough for them to hear it too. That may be very valuable later if you do have to shoot him, as it goes to your intent (self defense), and his intent.

And, of course, have someone call 9-1-1 and tell them that you are holding an attacker at gun point. You do not want responding police to shoot you.

Shooting another human being is not a natural act. It may disturb you afterwards, it may not. It may cause you criminal or civil problems later. You should think these things through now, not in the heat of the moment.

Being well trained with a firearm also includes practicing the art of talking to people that you are holding at gun point. Do not allow them to talk to you, as this is a classic distraction technique. Give them clear commands, one at a time. Tell them what will happen if they do something other than what you want.

You might also include training in when NOT to shoot. If your attacker, seeing your handgun, puts up his hands and turns and walks away, you might not shoot him. But you might keep the gun on him for awhile. Lots of factors to consider besides handgun make and caliber.