The 21-foot rule is more of a guideline (and may save your life)
As you walk out of a late movie with your date, a shady character steps into your path about ten feet in front of you. He produces a switchblade and demands your wallet. You know that in order to reach your wallet, your hand will swipe right past the concealed carry holster your trusty Glock 19 is nestled into, but could you level the weapon and fire before the assailant pokes you full of holes?
Chances are, you couldn't.
There's always room for debate within the tactical training community, as experienced (and often inexperienced) gunslingers develop their own unique approaches to engaging armed opponents. While many opinionated enthusiasts will subscribe to the idea that there's only one right way to train or fight, the truth is that the right approach is often dictated by the user's ability, training, and nerves.
Put simply, two different people could be put into the same set of circumstances and may use the same approach to try to get themselves back out of it, but because of the innumerable variables at play in any fight (whether we're talking fists or nuclear missiles), placing bets on a winner can be a crapshoot. That's why, when it comes to training to survive a fight for your life, it's often better to operate within training guidelines rather than the rules you may see published by those who assume it's their way or the highway (to hell).
One such rule that is really more of a guideline is the often-debated "21-foot rule." This rule was first posited by Salt Lake City police officer Dennis Tueller in an article he wrote entitled, "How Close is too Close?" Put simply, Tueller determined that an assailant armed with a knife or club could cover 21 feet in about 1.5 seconds, which is faster than most police officers could draw, aim, and fire their weapons from their hip holsters. This assessment produced two important tactical norms in the minds of many: the first is that a person may be justified in shooting an opponent armed with a knife or club within that distance, because there may not be time to adequately react if they chose to attack. The second is that once you're inside that 21-foot radius, your approach to survival will need to shift.
In the years since Tueller's article was published back in 1983, this rule has been debated, "debunked," re-debated, and incorporated into many training regimens… but it's important not to get too caught up in the figures. That 21-foot figure was really meant to be a rule of thumb, rather than a hard-and-fast rule, because shooters of different skill levels respond at different rates of speed, opponents aren't all the same speed either, and countless variables regarding the officer's equipment and the environment the altercation takes place in can all affect how quickly and accurately a shooter can respond with deadly force.
Likewise, for those of us that aren't members of law enforcement, relying on the idea that the rule is 21 feet can be pretty dangerous. Most casual shooters don't have the same training and experience with their firearms as police officers tend to, and it often takes longer to draw a weapon from a concealed holster than it does from the open-carry hip holster position employed by most police officers.
So does that mean the rule is bunk? Absolutely not — it just means you need to use a bit of common sense in the way you employ it.
If you pride yourself on your Wild West quick-draw skills, your safe engagement distance might be notably shorter than 21 feet. If you do most of your shooting at a relaxed pace inside your local gun range with a stationary sheet of paper standing in as your opponent, your safe distance may actually be quite a bit greater than 21 feet.
The exact distance isn't as important as the understanding that a gun isn't a guarantee of victory against knife-wielding attackers. In fact, inside the distance it takes to get a first down playing football, a knife can often be the deadlier option.
That information can help inform your approach to dangerous situations — like just handing over your wallet to that mugger that was inside of ten feet of you and your date. It can also help you prioritize targets in a multiple assailant situation.
If you want to know what your own equivalent of the "21-foot rule" is, it's simple: have a friend time you the next time you're training for rapid deployment of your firearm from its holster (in a safe and controlled environment). Slower than 1.5 seconds? Then your rule is further than 21 feet.