As a retired US Secret Service agent who took me under his wing once said, "it's all fun and games until it becomes a Two-Way Range."
(My ex was actually a better shot under benchrest no-pressure conditions; I was the better shot under stress like timers and Shoot/No-Shoot decisions in "Hogan's Alley.") This 20' goes back to what we call the Tueller Principle: a running man with a knife can cover a distance of about 20' and have it plunged into your heart in about two seconds, faster than even the great Quickdraw Artists of the Old West could unholster, line up and get in a stopping shot. (Named after the guy who did the research and coined it, Det. Dennis Tueller of Salt Lake City PD.)
In the Practical Shooting sports we even have a drill to simulate this... you and a buddy stand back to back on the firing line with your weapon holstered. When a timer beeps, you draw and fire as quickly as you can while your buddy takes off running like the Devil himself was trying to shove a red-hot pitchfork up his ass, stopping only when he hears your shot. The distance he stops from you is your personal "Danger Zone" under the Tueller Principle.
Amazing the things you learn, and the speed you learn them at, when the one you want to make babies with has their life depend on it, isn't it?