They have been able to use their weapon as a deterent in a potentially violent confrontation.
No, the defender does
not use his weapon as a deterrent. His opponent(s)
perceived the weapon as a deterrent. Semantics? No, it is a difference that the defender has absolutely no control over.
I drew one day to low ready in a situation where I did not need to fire immediately. When the two intruders fighting each other in my living room noticed that I was doing more than just standing there in horrified amazement, they fled the scene and began fighting each other again down the block. But I did not
use my weapon as a deterrent. I drew my weapon knowing I might have to use potentially lethal forced against a fellow human being. The intruders decided my weapon was a deterrent. I had nothing to do with their decision even though I was grateful for it.
21 feet and you get cut ...
Sorry. From real world experience, I've always taken the Tueller Drill with a couple of pounds of salt. I'm not that fast. Average, if that.
Was talking with a customer in his garage/shop one night about the imminent repossession of his washing machine. The conversation was amicable, no upset, and no indication things were about to go south. We were no more than five feet apart. There was a tool chest to his immediate right with a row or bags of nails across the top compartment just above waist level. Suddenly he grabbed the sledge hammer handle concealed behind the bags of nails and swung downwards to my skull while taking one step forward. Big problem for him, though, as my skull wasn't there any more. I had gotten off of the line of attack by taking as big a step as I could with my right foot to my right rear oblique (45 degrees). I was now four feet away from the sledge hammer handle, five or more feet away from him and had swept my cover garment, hit the thumbstrap, and was pulling the pistol. I would be pressing the trigger the instant I had a sight picture. My assailant realized that I was drawing a handgun from my actions, realized I'd have it out before he could reach me, and dropped the sledge hammer handle.
He stopped his attack and I stopped my defense. I didn't
use my weapon as a deterrent. He was deterred at a point where I could halt my defense. If he had dropped that handle as I acquired a sight picture, he would have probably been shot at least once.
But if you train to get off of the line of attack as a knife or impact weapon gets close then the distance limits of the Tueller Drill don't hold for you. The Tueller Drill really is only relevant if your feet are nailed to the floor or you have neglected to train as you should.
According to the Tueller Drill, my skull should have been crushed by the sledge hammer handle. Not even close though it would have been if I had followed the scenario of the Tueller Drill and just stood there while trying to draw my pistol.
Now you won't learn how to get off the line of attack from an assailant using a weapon at a martial arts school that doesn't train with weapons. All martial arts schools are not equal. This is not a dig at martial arts schools that don't train with weapons. It's simply noticing that if you train with different foci then you get different results. I was fortunate enough to train in a system where not only did the students train with hand weapons but you also trained where the opponent had the club and you didn't.
If you stayed in the fight, you did a lot of evading until, and unless, your opponent made a mistake.
The Tueller Drill's 21 foot rule is true, but only if you just stand there waiting to get cut while trying to draw. Caveat: the time to figure out how to effectively evade a knife or club is
not when you're frantically trying to draw your pistol before the knife or club makes contact with your precisious butt. Believe me, this will make for a bad outcome. For you.
I can't give pertinent advice on brandishing as there are no such laws in my state nor have I ever lived in a state where there were such laws. I will repeat, once again, know the laws relating to the use of handguns in your state and in any state you will visit.