I think there was a study that showed 21 feet was the minimum distance that you had better "make up your mind" to stop the threat against an knife or club. If you are face to face and have something pulled on you, there is no time. You either shoot or run.
Sigh
Here we go ... again.
The Tueller Drill (named for a police officer named Tueller, who developed it) is the source of the so-called "21 foot rule." If you're going to keep talking about it, it would perhaps be a good idea to understand what it represents.
First, it applies ONLY to uniformed police officers carrying in duty holsters. Second (which I'll get to), it is obsolete. Here's the deal:
Lt. Tueller was an instructor and he wanted to find a way to teach new recruits a bit of the "sixth sense" that savvy street officers develop over the years. One of the issues was "When do you draw?"
So he started with some timed drills. He had anumber of his officers draw and fire on a target. The average time elapsed from the command to the shot was 1.5 seconds.
He then had some guys armed with rubber training knives rush officers from various distances, to see how much distance a knife-wielding assailant could cover in that 1.5 seconds. Turned out that 21 feet was the distance. That's where 21 feet came from. What it taught his officers was that,
when faced with a knife wielding assailant, if you allow the guy to approach within 21 feet you are behind the curve, so you need to draw
and go to low ready. Otherwise, if the guy attacks you will not have time to draw and fire before he sticks the knife in your chest.
Caveats: As noted, the drill only applies to uniformed officers. Specifically, it applies to uniformed officers in Lt. Tueller's department, wearing whatever duty holsters they wore back then. Tueller himself has commented that the entire drill is now obsolete because newer retention holsters require more time to draw, thus negating the 1.5 seconds and thus negating the 21 foot "rule."
This does not apply in any way to "civilian" CCW holders carrying in concealment, unless you know from practice and experience that it takes you exactly 1.5 seconds to recognize a threat, draw, aim, and fire. Dunno about any of you, but I think that time would be quite a bit longer than 1.5 seconds for me. Which means that an armed adversary is an immediate (potential) threat to me well beyond 21 feet. How much beyond? I don't have a clue.
I have seen people on this forum mis-state the Tueller "rule" as saying that it means you MUST shoot if a threat is within 21 feet. No, it does not say that. The laws regarding use of deadly force are not suspended if a threat is less than 21 feet from you. Even if the threat is only 10 feet away -- if you reach for your piece and the threat decides to wish you a good day and walk ... you have no cause to shoot.
Bottom line: The 21-foot "rule" is NOT a rule ... it was a rule of thumb, not an absolute command. And ... it is obsolete. And ... it was only ever applicable if your time from threat recognition to shot was exactly 1.5 seconds.
We now return you to your regularly scheduled programming ...