Oy vey.
OK. First: in MOST states the basic rule one is that you can shoot somebody "when in fear of losing your life or suffering great bodily injury from criminal attack". That's the most common basic guidelines. Some states add a "duty to retreat" thing in there, many (most?) states allow you to shoot (or more technically, "use deadly force" so a knife/hammer/etc. is legally no different) in defense of others.
If you haven't read up on what the rules are for your state, you're Doing It Wrong[tm].
Second, in the VAST majority of states, there's very little legal distinction between pulling a gun and shooting. In other words, if you can legally do one, you can do the other.
There's odd variants between states. For example, Arizona just passed a new rule that says when a person threatened displays a holstered handgun, puts their hand on it but doesn't draw yet, a lot of leeway goes to the defender. They'd still best have a reason but the standards are lower as long as they don't clear leather (actually draw). This is a rare type of rule!
In ALL states, the moment the threat evaporates, you lose the ability to shoot. This could be because you did indeed shoot them sufficiently, or it could be because you didn't manage to get a shot off before they screamed like a little girl, wet their pants and ran off at full tilt boogie. This happens a LOT. It's called a "chase off" or the like and if you can articulate that this is indeed what happened, you SHOULD be legally in the clear. It is after all the most common thing that happens when a gun gets pulled in self defense - by at least 90%, for ordinary citizens OR cops. Been there, done that, watched various lunatics retreat at speed and I've never been arrested let along charged with a violent crime - ever.
Now. This leads to our next problem. "If all you have is a hammer, the whole world looks like a nail". If a gun is your first, last and only self defense method then guess what? We're back into Doing It Wrong[tm] territory - what you're doing is WAY legally risky unless you're elderly, infirm or a small female.
Let's go back to the most common legal standard again: "fear of losing your life or suffering great bodily injury". If that fear comes JUST from the fact that you're packing and they could get the gun, then you run smack into another problem: your "fear" must be based on what a REASONABLE PERSON might fear in the same circumstances.
In other words, if your "fear" of somebody comes from the fact that they're actually an alien slime-being from Arcturus IV in disguise, it ain't reasonable and you're headed for either jail or a loony bin. As it should be.
Well guess what? Your 12 jurors may not be gun-toters even in a rabidly pro-gun state like Arizona. One or two might be but their "words of wisdom" (assuming they have any) could easily get drowned out and they cave in on "guilty".
If you are carrying a gun, you need to have a plan "b" non-lethal form of self defense in case the threat doesn't rise to an obvious level of "they're gonna kill me or massively screw me up".
Not confident in your unarmed combat abilities? OK. They make this stuff called "Pepper Spray", also known as "Kung Fu In A Can". Get a big one (4oz or so) with a fine-stream nozzle and a "spiciness rating" of at least 1.5 million SCU (Scofield Heat Units). It should say what it is on the can - if not, it's junk.
Give 'em a faceful of that first. Doesn't always work but hey, at least you tried. If he goes completely ape**** bonkers and you still need to shoot him, well, you still come out looking better. And the stuff DOES work a lot of the time.
In ALL cases where you choose not to use a gun, keep it under wraps if possible!!!!!!!!!!!!!!!!!!!!!!!!!!
In my case, my unarmed abilities are at least a bit better than average (being 6'4" and 280+lbs helps) and I carry a good-size AA-length flashlight as a "kubaton" type weapon that I've had some training in. Given one large assailant or maybe two small ones (depending on how crazy they seem) I'll try that first. The *instant* I think a weapon is involved on their part, or I see signs an opponent is drugged up, a very large 357Mag is going to join the party loading with six rounds that are each about 1/3rd more potent than anything the cops carry. And they'd best do the "scream and run" thing REALLY really fast.
Appendix: things that can cause you to be "extra afraid" of an attack.
1) Multiple assailants.
2) Much bigger assailant.
3) Assailant's motive - a robber for example is likely to be much more out of control than, say, somebody else at a party who doesn't like you hogging all the nacho dip or whatever.
4) Assailant's motive - HATE CRIMES. Even in states that don't have a "hate crime" law per se, most people (even judges/cops/prosecutors) understand that when an assault happens right after the assailant screams words like "nigger" or "faggot" or whatever, the likely violence level goes way, way up.
5) One or more assailants has a deadly weapon. Probably the most clear-cut situation and among the most common.
6) Assailant has just delivered a major beat-down adding up to "great bodily injury" to somebody else and is now turning in your direction!
7) Indications that the assailant is either crazy or on drugs.
8) Assailant has already delivered great bodily injury on you!
9) Due to infirmity, age or handicap, you have "extra reason" to fear getting punched. Examples may range from "recent head surgery, skull isn't yet fitted together very well", "known bone density problems such as Osteoperosis", "previous joint damage" or the like. "Bonus points" if it's obvious you've got issues (braces/crutches/wheelchair) and the assailant is hence an obvious predator looking for "prey".
10) When the assailant(s) are animals, the standards are much, MUCH lower! You still can't shoot somebody's dog for no reason, but it doesn't take a whole lot of aggression to give you cause to shoot. (You still have to worry about danger to bystanders - what's your backstop? That's a whole 'nother thing!) Wild animals, the standards are even lower and if you suspect something is rabid, you can generally go after it actively even if it's not attacking...but again, backstop and "shooting in urban environments" dangers need to be factored in.
There may be others but that covers the majority.
Case History 1: Gordon Hale
Mr. Hale was the first Texas CCW holder to ever shoot anybody. He killed a much larger, younger and crazier assailant. Hale and this nut accidentally bumped cars in a very minor way. Hale pulled over expecting a polite exchange of insurance or whatever, and rolled down his window. Younger nutcase started pounding him in the face through his window, with Hale still seated. Broke Hale's face, literally - detached his retina and broke his cheekbone, and then tried to drag Hale out through his own window. Hale grabbed his Glock, fired one shot to the assailant's chest, killed him dead. No criminal charges. (Note: ALL Texas shootings go to a grand jury, which no-billed Hale.) This was a "type 8" in my appendix above - the fact that Hale took serious damage and reasonably feared more helped a lot in the legal aftermath (criminal and civil court - assailant's family didn't get squat).
Case History 2: Harold Fish
Fish, like Hale, was an older gent in his 50s, walking in a wilderness area. Fish was charged by two dogs. He fired a warning shot or two into some dirt - remember, wilderness area meant far less threat to bystanders, and dogs meant his "right to shoot" standards are a lot lower so he's in good shape legally so far.
The dogs ran away.
The dog's owner however turned out to be a complete nutcase with a history of attacking people who criticize or threaten his often-out-of-control dogs. Dog's owner charges Fish, windmilling his arms and screaming in a crazed fashion per Fish. Dude is larger than Fish. Fish shoots him dead, one round. Tries to do some first aid, goes and gets help, no good, guy dies.
Fish was convicted - at first. The trial court erred in blocking evidence that the assailant tended to behave in exactly the fashion Fish described - right down to the crazy windmill arms attack, the yelling, etc. Other witnesses who had seen the same stuff before in other attacks were barred from testimony. Per the Arizona Supreme Court, this was a major violation of Fish's civil rights and he's free pending a decision on a new trial last I heard. It's more or less impossible that he'll be convicted. More details at:
http://www.haroldfishdefense.org/
Lessons: this was a combination of "crazy assailant" and "larger assailant". Fish was lucky in one sense that this guy's crazy behavior was on record but on another level, nutcases who do stuff like this usually have a track record and as the AZ Supremes noted, that track record is admissible in court IF it's to prove that the story Fish told the cops post-shooting was in fact accurate. In other words, you can't use the shot-up guy's past as justification to shoot him now! You CAN use the shot-up guy's past as evidence that he did the same damn thing in this case. There's a fine distinction between between these two uses for that data and the Fish case illustrates the difference.
Finally, Fish might have been better off using pepper spray on his *human* attacker, but he had an extra problem: the two dogs are what started the problem and trying to gas two fast-moving dogs at once would have been a stone-cold *****. Therefore Fish very properly pulled a gun, did a warning shot (legally justified, esp. since this was wilderness) and then only once the gun was out was he confronted with the nutcase. Putting the gun away and switching to pepper would have been risky on two counts: it would have taken extra time, and would have told the nut that Fish had a gun on him available to be taken away. (An argument can be made that the warning shots weren't very smart. I disagree under the circumstances, esp. since it actually worked great on the dogs!)
I AM NOT A LAWYER. THESE ARE MY PERSONAL OPINIONS. IF YOU'RE CARRYING A GUN AND HAVEN'T READ A GOOD SUMMARY OF YOUR STATE'S DEADLY FORCE LAWS,
YOU'RE FRACKIN' CRAZY.
I've had the act of handing off a gun do the job of being an effective deterrent.