It is not the 21’ rule, that is a reference to the Tueller Drill that shows the reaction time needed to stop a threat and the distance an attacker can cover in that amount of time. This USCCA thread discussed that drill. This link explains that it was just the average distance/time in which it is a tie, wherein the attacker is upon you and you are able to put a shot on target.
The significance of the time factor is based on the reasonable standard that a person who’s trained in proper pistolcraft should be able to draw a handgun and place two centered hits on a life-size silhouette at seven yards in about 1.5 seconds. Before I go any further, I want to point out that both the distance of 21 feet and the time factor as addressed in Tueller’s original article, were both approximations based on training experience; nothing more.
This is directly from “the horse’s mouth” an article by Dennis Tueller from the March 1983 issue of SWAT magazine.