By now everyone interested in firearm self defense has heard of the "Tueller Drill" and might even have done it Force on Force (if you haven't you should, under qualified supervision of course. Lot's of gunfightin' nonsense get cleared up real quick). So anyway, here is the original article by Dennis Tueller that started it all, "How Close Is Too Close".
http://www.theppsc.org/Staff_Views/Tueller/How.Close.htm
http://www.theppsc.org/Staff_Views/Tueller/How.Close.htm