One thing I don't understand from your description: If a round is in the chamber and the hammer is cocked, what good does it do to grab the barrel? I believe if the hammer hits the firing pin and the firing pin hits the primer, it wouldn't matter if you were applying pressure to the barrel or not. The gun will fire, no?
Now if you can block the hammer with your thumb, either preventing it from moving backward into the cocked position or preventing it from striking the firing pin after the trigger is pulled, then you could conceivably ruin some gunman's day. However, I don't think I'd want to try this trick as too much would depend not only on your own reflexes but also on his.
I've seen something similar with revolvers . . . If you can grab the cylinder, preventing it from rotating, the person holding the gun can't pull the trigger . . . However, if the gun is fully loaded and already cocked . . . Watch out!
PS, I'll also watch this thread with interest now to see what others more knowledgable about modern handguns than me have to say about all this! :-)
You're not grabbing it- you're pushing against it. And keep in mind, I don't have any idea if it works with other automatics.
Pushing straight against the barrel of the 9mm Military sidearm, pushes the barrel backwards- towards the hammer. Somehow, it moves the slide just enough so the trigger- hammer mechanism doesn't function. If you have a 9mm like the one the military uses, give it a try (unloaded of course). If you push against the business end of the barrel- that is, if the gun were pointed at you and you stuck out your palm (in a "halt" posture), placed your palm straight against the barrel and pushed directly against it- you can actually see it interferes with it. It pushes the hammer back a little bit further than it's supposed to be- whatever it does, it prevents the weapon from discharging (no matter how hard the guy squeezes the trigger) as long as you have it pushed back.
If you were quick enough, I suppose you could unlatch his magazine and then if you could force him to discharge his one remaining round, he'd be screwed. I found just using that method to push the barrel away from my torso/head and then using my free hand to strike seemed to be a pretty good tactic- assuming the guy was going to kill you anyway, having a hole in the hand is definitely better than one in the gut and the worst that happens is you're not quick enough and he plugs you anyway.
If you can, try it out safely. If you don't have one (9mm), go to a pawn shop or a gun show and see if you can find that model and give it a try. The magazine doesn't have to be in it. Try it either way, hammer cocked or hammer down. I don't know how to describe why the hammer can't be released, but it's like it's pinching it/putting it in a position where the trigger doesn't work. But you're right, if you didn't ID the gun correctly, you'd be screwed. We figured it would actually only be useful against another US Soldier because the probability would be highly in your favor that it would be a 9mm (against the MPs maybe?)