The "21 foot circle" is itself a "myth."chasfm11 wrote: ...It certainly appears that their use of their guns has to be determined beyond the 21 foot circle. ...
Tueller's article in the March 1983 SWAT magazine used the distance of seven yards. He doesn't state why he uses this distance, but I am pretty sure it was because this was considered the distance (or less) at which most police shootings occurred, or at least it was the (one of) the distances police officers trained for. He stated that those who practiced pistol craft were of the mind that a good shooter should be able to draw and put two shots center of mass at that distance in 1.5 seconds.
He further said that through some experiments, he determined the average adult male (armed with a knife, club, or other contact weapon) could cover that distance in the same 1.5 seconds.
His conclusion was that if the attack starts at seven yards (twenty-one feet), the attacker is already with in the "Danger Zone" and the defender is quite possibly going to get hurt or killed if he doesn't do something to change this. Thus he goes on to recommend a number of things, like movement, cover, obstacles, drawing weapon as soon as danger is sensed (or "determing use of the gun beyond the 21 foot circle"), etc.
There was nothing magic about 21 feet, and in fact he concluded that this could in fact be "too close."