I've seen the results of someone stopping the forward movement of the hammer with the web between the thumb and the index finger. It will work! But be prepared to lose a chunk of skin and you better be moving because you won't get a second chance. I've been told that if you rush the individual with the firearm that chances are they will miss when shooting at you, unless (make that a BIG UNLESS) they practice shooting targets that move at them or they could just get lucky. I'm not so sure I'd want to play that game. I guess it would depend upon the situation. In my younger days I've had one gun and two knives pulled on me. And in each case I became a "Yes Sir!" kind of guy mighty darn fast. As a NRA Certified Instructor in Home Defense, I tell my students to keep the gap between the "Bad Guy" and yourself as big as possible, just to prevent the intruder, rapist or what ever from trying to take away your firearm. And if you pull your firearm you must absolutely be mentally prepared to "shoot to stop" the person coming at you. Plus I also advocate plenty of shooting practice.
I just hope that none of us ever find ourselves at either end of a firearm during a bad situation.
Yours in Aikihood