HOW would some super duper martial artist keep the gun from going off? I know when I practive defencive shooting I have my finger wraped around the trigger, not like when you target shoot and have just your fingertip on the trigger. I can see you MABEE doing it with a DA gun, but with anything single action, the gun is going to go off.
Well, first of all, it doesn't matter if the gun goes off....most disarming techniques involve (as a first step) getting the gun pointed away from you. Maybe not so good for innocent bystanders, but it'll keep YOU from getting shot.(although most disarming techniques actually involve moving the gun upward, and away, so the risk to bystanders is minimal) And, of course the second step is disabling the guy with the gun. Most disarming techniques are counter-intuitive, in that you aggressively move TOWARDS the attacker (gun holder) and get inside his ability to shoot you. (same applies for knives). Disarm (or render weapon useless) THEN disable.
I used to teach self-defence classes (mostly to women, when I was heavily involved in martial arts) at the college I went to. One of the demonstrations I always did was to allow a student to hold a knife about 1/2 inch from my throat. And tell them to jam it in me as soon as I moved (from a position with my hands up and to the sides). While we used a butter knife, I would've had no fear of using a VERY sharp knife. We used the butter knife, because it would always end up flying across the room when I disarmed them. Again, there is NO WAY you can react as fast as needed. I also taught them to move in the middle of a sentence (pleading for mercy or WHATEVER)...this lulls the attacker in to a false sense of security (for some phsycological reason, no one expects you to attack while you're talking).
Now, some one that's REALLY fast, and knows exactly what they're doing, can probably snatch the gun away before you can fire EVEN IF YOUR FINGER IS ALREADY ON THE TRIGGER. Try holding onto ANYTHING while someone is bending you fingers backwards---the automatic (really autonomic) reaction to the pain is to release whatever you're holding. Again, if you manage to fire, so be it...but the gun will now be pointing in a different direction (possibly at YOU.)
So, in short (can't really explain it without demonstrated it), you first control the weapon, as you move inside the effective radius of the weapon, rendering it harmless, then follow with a disabling (or fatal) blow. (I usually taught it with a throat strike...fatal (in a coupla minutes if done properly, but definitely disabling, even if not).