1. If you shoot the knife guy at less than 21 feet away.. you didn't win; you very likely traded a knife wound for a gun wound.
2. Your ability to stop a charging guy with a knife if you do shoot him depends on where you hit him. Results will vary from him dropping like a sack of potatoes to acting like nothing happened.
3. 21 feet is the beginning of the danger zone if you have a gun and the bad guy has the knife. It is still hard to shoot him and not get hit with the knife at 21 feet if you are more or less ready. If you are not ready, you most likely will receive a knife wound. There is a difference between the Tueller Drill and getting ambushed by someone with a knife.
4. I have seen very good gun guys do the 21 foot rule. They depend too much on their shots to protect themselves. Do you want to shoot the bad guy with the knife or protect yourself from getting stabbed?
5. Try the 21 foot rule using concealment.... it might be different.
6. When you visualize the 21 foot rule, do you visualize a 3.5 inch folding knife or 3 feet of razor sharp sword? It might change things.
7. The adept will know that the concept of timing is important to surviving in regards to the 21 foot rule.
8. There is the myth that the guy with the knife cannot adjust while running. He can. You move... so can he.
More to come....