I think we always need to be AWARE of what 21 feet represents. Once the opposing force has entered our 21 feet radius, we need to start the kill-don't kill mental calculations. If they enter the the 21 foot radius at a dead sprint, we are at a disadvantage because they have made their kill-don't kill mental calculations and we are left with less than 2 seconds to make our calculations, overcome our instincts and our fears, draw our weapon, point it, and squeeze the trigger.
I had a shoot-no shoot encounter inside 21 feet earlier this year. I had time to calculate so I pulled the gun out of its holster and, without pointing it at anyone, made the opposing force aware of their disadvantage. They didn't waste a lot of time increasing their distance from me. They increased their distance by a couple of miles in fact.
Recognizing that a threat was less than 21 feet and recognizing the disadvantage that that created for me allowed me to make several mental calculations and to arrive at a solution that was appropriate for the situation and which didn't get anyone killed or sent to prison (although I did have to explain myself to a judge due to the nature of this incident but not because of the gun specifically).
Situational awareness was my ally. This is not the first time that situation awareness has saved my ass. We need to be AWARE of the 21 foot rule and the SITUATION it creates for us.
I personally would have my weapon unholstered and in a shooting grip long before the danger got within 21 feet. Enter the kill circle and you’re already looking down a .45-caliber smokestack.
Your move, tough guy.