“Double Tap” is a slang term that I don't believe has a precise meaning. They don't use the term at Gunsite, which has been teaching for a long time now. Most commonly, I believe it means two fast pistol shots.
Similar terms which do have precise meanings are “controlled pair” and “hammer.” But some students of this combat shooting business are now suggesting that the very idea of delivering two shots to the target—whatever you call it—is fallacious teaching.
They argue that programming shooters to perform this technique may be wasteful of ammunition, because one shot may very well get it done. More, they contend that it is better to teach the student to shoot until the threat is resolved. That is a very good point and one with which I agree in principle. However, I also believe that what you should do in a life-threatening emergency is never simple.
Since a handgun of any caliber is the worst possible firearm for personal defense, but nonetheless the only one you may have with you in an emergency, its use should be optimized. For that reason, I argue that every beginning shooter should develop the ability to deliver two fast shots to the center of mass as a basic response.
It can be demonstrated that two shots more than double the terminal effect of a single one. If more are needed, two more is not out of the question, but the general rule is two shots per customer. It isn't done to increase the possibility of a hit, but rather to increase the probability of stopping the fight quickly.
