I disagree. Most states have the phrase "or great bodily harm" listed in the rules. If you don't think a punch can do great bodily harm then you are mistaken in my opinion.
A black eye or bloody nose, the injury
most typical and associated with a blow to the head are not considered great bodily harm. Most punches to the head won't result in your dying.
That is fact. A minority of blows to the head placed by great luck or a skilled fighter can result in great bodily injury or death, but in the eyes of the law this exception to the rule does not negate the rule. Cases of self-defense are judged by the Reasonable Man Doctrine, whereby a reasonable and prudent person would act exactly as if you had. Again, if you were someone with great physical ailments, you can legally use lethal force since getting punched in the head, with your medical conditions, might guarantee a kill.
Using the same logic construct you've created, can we kill someone because they tried to kick me in the leg? A shattered femur is considered great bodily injury, and an extremely lucky blow or one placed by a martial artist can likely break my femur. Can I shoot someone on a golf course that struck me with a golf ball? A golf ball, if struck just so, can cause severe head trauma.
By taking all these extremes based on "what if" rather than "what usually is", you're taking major legal risks. Yes, perhaps in a non-existent world that you envision, not tied up by such much monetary and legal hassle after-the-fact, you can go about shooting people if they so much as take a swing at you. However in the real world, you might survive the encounter, but not the aftermath in the court system. That is ultimately up to you how much risk you take.
You can disagree with me, but what I stated is based on training courses and what the law already recognizes and follows, not opinion. Taking "great bodily harm" in itself is absolutely worthless outside the context of the law and all the other bits and pieces that incorporate with it. Very rarely will a healthy male defendant using self-defense in the manner you describe make it off the hook. Cutthroat prosecutors already chase after anyone using firearms successfully in self-defense. Under the circumstances you've created, it'd make their job that much easier.
"This poor man went to take a slap at Chuck Spear's head. So Chuck Spear shot him in the face". That's going to be damned hard to explain.
Your explanation of "well, his punch
could have killed me" is not good enough.
Now, if you're a woman, 9 time sout of 10, disparity of force exists against a bare-fisted man and you can blast him away. If you are handicapped or have physical ailments, you can play the eggshell skull doctrine which illustrates a disparity of force. You, as a healthy male of average size and build going up against another bare-fisted healthy male of average size and build does not, under the reasonable man doctrine, give a disparity of force unless you can come up with some very obvious and additional exculpatory evidence that suggests ability was there.
Look into the definition of great bodily harm, serious physical injury, and the concept of ability, opportunity, and jeopardy, and the reasonable man doctrine. You cannot form an airtight legal defense by saying that his punching you was "great bodily harm". You'll fail epically.