The Tueller Drill, a self-defense training exercise, says that the average bad guy with an impact weapon (like a knife or club), can cross 21 feet in about 1.5 seconds. Which is roughly the same time the average person can draw a firearm from a holster and put two shots on target. At distances less than 21 feet, the good guy with a gun will typically not have enough reaction time to draw his weapon. This should encourage the good guy to do what he can to create more distance between himself and his adversary. The Tueller Drill, in no way, implies that you may not shoot someone unless they are inside 21 feet. It is not a legal standard which mandates a minimum distance of 21 feet before you can defend yourself. It is a training standard which encourages good guys to create space between themselves and the bad guy. At distances greater than 21 feet a typical person with a handgun can draw the gun and place two shots on target before the bad guy can cover the distance and strike them with a melee weapon. To better protect those you love practice draw speeds and learn the situational awareness needed to keep greater distances between yourself and the bad guy.