This about J frames and other "belly guns" is a popular belief among those too lazy to practice with their small guns.
That's it in a nutshell. People too lazy/busy to practice.
I like to shoot. I retired from 20 years of LE about the same time I retired from the NG. I ran the marksmanship unit for the AK NG and was a firearms instructor with the Anchorage Police Dept. I like to shoot.
When I retired I wanted to move back to Wyoming and buy a place where I could shoot in my yard any time and as often as I wanted. I did and I do shoot quite a bit. I carry a 642 in my pocket (no holster) and I practice with it A LOT.
I've always been the type who walks around with my hands in my pocket. I still do.
I walk around my back yard range drawing and shooting shootgun hulls laying around. I even have shot some ICORE and Bullseye matches with my 642 (didn't win, but I've learned to shoot my pocket revolver).
As to the 21 rule; Never paid attention to it. Might as well be the 100 yard rule if one doesn't practice.
But I would consider the 21 foot problem being speed more then accuracy. How fast can a baNdit cover 21 feet vs. how long it takes you to get your revolver in action.
Using a shot timer, and my hand on my gun in my pocket, I like to keep my first shot about 1/2 second, hitting the "kill zone" in a target at 7 yards.
I just reciently got back from a road trip to Portland to visit my daughter. We went shooting. She asked if I was really as fast as Grandma says. (Grandma being wife, her step mother).
I handed her the shot timer and said "lets see". She got behind me and hit the button. When the "beep" went off I drew and fired. .45 seconds.
Another thing I like to practice, and deminstrated for her is drawing and firing when someone has the drop on you.
I had my daughter point her gun at her target, I told her to get ready, finger on the trigger, safety off, (remember she's pointing at her target), When she sees me start to draw she was to fire, the ideal was to see who gets the shot off first.
I ask her is she was ready, yeap, I told her I was going to draw, she said OK, I'm ready, I fired, she said she wasn't ready. I did this three times, and always got the shot off first.
This is while she was knowing what I was going to do, few bandits expect people to draw when the bandit has the drop on them.
Its really simple, its faster to act then re-act.
There is no trick. I'm 64 (will be 65 in 10 days), I'm not in shape. I AM NOT A PISTOL SHOOTER, mainly a rifle shooter, I have to work hard to keep half way dicent with a pistol or revolver.
This can be seen if you go to the matches we have, I shoot some sort of action pistol match every other week. Don't win much, I'm too slow, mainly in the reloading department. Accuracy isn't too bad, but I'm slow moving from target to target.
I do dry fire a heck of a lot.
The point being, its not 21 feet, its whether one wants to take the time and effort to practice. I'll admit, if I lived in town and had limited access to a range, I probably would be too lazy to go often.
As to where to shoot. Read Captain's Fairbairn - Sykes' book "Shoot to Live".
Fairbairn was the chief of the Shanghai Police dept when gangs ran the city. He was involved, one way or the other, in over 600 shootings.
Two things you will get from the book, bandits, when shot, even with a 45 seldom go down. There is no such thing as a guarinteed killing shot with a pistol.
The second thing is if one is "gut shot" though they may not die or stop right away, they normally drop what they have and grab their stomach.
We all know that if we've been hit in the stomach.
It ain't about the gun, its about what you do with it. Practice is the name of the game. You don't have to be young and in shape. I'm not. Besides being old, I have several stints in my blood vanes and have COPD. I ain't gonna last long in a fight, but I bet I can last 1/2 second.
I spend a lot of time attending my granddaughters' sporting events, vollyball, basketball, track, etc. Those young ladies are in shape and fast. The fastest of the lot can't cover 21 feet in 1/2 second.
As a side note, and un-related. I'm a firm believer that SD pistol/revover practice should be with one hand. In my LE career, and my time as an infantryman in Vietnam when I used a revolver or pistol, I can't remember a time where I was able to use two hands, something was always in the other hand, flashlight, door knob, mirror, something.
PRACTICE IS THE NAME OF THE GAME.