Yeah the 21 foot rule is well known, Sheepdog - but keep in mind that range was developed with officers wearing guns on duty belts. It does NOT take in to account having to draw a concealed weapon with a cover garment in place! Drawing a concealed handgun may take appreciably longer.
If I recall the distance was:
Under 15 foot - you're stabbed 100% of the time.
15 foot with hand not on gun at start - you are stabbed well over 50% of the time
15 foot WITH HAND ON GUN at start - one in five can get a single shot off before being stabbed
21 feet (with or without hand on gun) - most folks can get two COM shots off on the advancing individual.
Now add in a cover garment? You might realistically need 25 feet or more, even if you are good. And if you aren't? And you snag on something or fumble? (Maybe you're wearing a winter coat over everything...)
Keep in mind that "21 foot rule" was also
designed for officers confronting a dangerous subject. They *KNOW* they might be in danger and they are fully aware of the bad guy and focusing their attention on them, their body movements, etc. In other words, they are clearly in the red zone on thinking and their body is already amped up and ready to move to defend themselves.
But most of us are not officers who are knowingly confronting a dangerous subject.
We're just normal folks going on about our daily lives.
By the time you realize you are in a fight, realistically, the person is already swinging at you or rushing you. Your first visual indication that a person is a threat might be when the guy standing next to you in the line at the gas station puts a gun to your head. Or the person you walk past on the sidewalk decides "hey, you're getting mugged".
No, facing someone down in a parking lot at 20+ feet away .. that's not likely to be how things will go when you are defending yourself. Your first clue that you are getting attacked very well might be GETTING HIT BY SOMETHING HARD THAT YOU DID NOT SEE COMING.
And then what? What if the opening plays out like it did when I was attacked and you are nearly fully pinned on the ground with a bigger guy on top of you?
Guns are great for self defense tools BUT they are only ONE SMALL PART of the equation. They will work in certain situations, and in others they will either be completely useless OR may very well pose a threat to YOU.
Folks who say they are too old or whatever for self defense, you really need to stop and rethink that. An old person can still crush a mans skull with a sturdy cane. An old person can still hit vulnerable soft spots that will make an attacker not be able to attack you anymore. You don't have to be strong, or fast, or spry to jab your finger in someone's eye or smack them in the nose to cause their eyes to water up. You can still hit their windpipe, or put your knee their genitals. Even if you can't lift your leg waist height, you can still kick them in the knee.
If you go through life ONLY prepared to use a gun for self defense, you are setting yourself up for a real bad time if you find yourself in an encounter where the gun is practically worthless.
I'm a concealed carry instructor, multi discipline certified NRA instructor, etc, etc, but even so, I will be completely honest with folks and say "the gun is not your first tool in a fight. YOU are the first tool in the fight. The gun is only there if the situation warrants it, and it very well might not."
I make sure every last student that comes through my classroom has a VERY clear understanding that wearing a gun does not make them invincible or suddenly turn them in to Superman. I make sure EVERY last student leaves with the words "make sure you also pursue hand to hand training" ringing in their ears.
Knowing how to handle yourself in a fight that starts out with a grapple is VITAL to any fight. Your very survival may depend not on how well you can put lead on target, but rather, how good you are at buying yourself
time and distance.
If you can't realistically buy yourself any time or distance, if you lack the knowledge for handling a hand to hand encounter, the gun you carry may very well be worthless to you someday when it comes time to defend yourself. If you can't win that time and distance that you need to employ it, you won't be able to.
Anyway, off my soapbox.
Go learn some hand to hand skills - it will complement your gun-fu nicely.
ETA: My 83 year old grandpa may be old and slow, but I still wouldn't want to take a right cross from the guy. He'd plant my butt right there if he connected.