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Dennis Tueller, a Salt Lake City police officer and firearms instructor (since retired), asked just this question. Uniformed officers are routinely faced with impact weapon bearing suspects. So it’s natural for Tueller to wonder how far away a suspect can be and still use an impact weapon against an officer before he could defend himself. To answer his question, Tueller ran a bunch of empirical studies. Which is just a fancy way of saying he ran a bunch of students through the exercise that would later become the Tueller Drill. Tueller learned that most officers can get a service pistol out of a holster and engage a threat with center-mass hits within 1.5 seconds. So the question then becomes, how much distance can a bad guy cross in 1.5 seconds? Timing a great many students running from a standing start, Tueller learned that someone can go about 21 feet in 1.5 seconds. So 21 feet became the “Tueller distance,” or the maximum distance from a police officer a person can use an impact weapon against the officer before the officer can shoot them. The Tueller Drill is often referred to as the “21 foot rule,” or the “7 yard rule.” This really obscures the real take-home message of the Tueller Drill. The value is not some particular distance. What matters is your “Tueller distance.” People’s draw speeds vary. Your Tueller distance will be greater or less than 21 feet depending on your ability to get the gun unholstered and pointed center-mass. The real lesson of the Tueller Drill is that someone armed with an impact weapon has the opportunity to use it at a far greater distance than most think—and certainly much greater distances than a juror might have otherwise thought. If you imagine the length of typical American parking space, and add another three paces, you’ll be right about at 21 feet.
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Andrew F. Branca (The Law of Self Defense: The Indispensable Guide to the Armed Citizen)