In any disarming technique, you will be directing the muzzle away from you. That's the most important thing. If you have to grab the slide to do that, and if he subsequenty pulls the trigger and jams the gun, so be it. You're not going to rely on it or wait for it. You're going to continue what you were doing. Disarming, striking, grappling, and fighting for a better position than your adversary.
I would not dream of attempting this stunt in order to give me time to draw my own gun, unless I was some kind of special at quick drawing and point-shooting moving targets at close range in awkward positions while being pushed/pulled off balance. As soon as you grab the gun, he is going to react... likely he is going to pull the gun back. HARD. If your other hand is drawing for your gun, it is NOT controlling the situation. He has two hands and his gun is already out. If you are both untrained, you'll both end up with two hands on his gun and/or each other, and trying to stay away from the muzzle end. You will want to keep him as close to you as possible so you can maintain a grip on the gun/wrist/arm, all the while trying to keep your balance and throw him off of his. You might get opportunities to throw in headbutts, elbows, knees, or kicks, in the meantime. This will assuredly require two hands.
If you are trained and he lets you decisively and firmly grasp the gun in a positive grip in a moment of surprise, you will probably disarm him before he knows what happened. But if that fails, you are in the same situation. Two guys, one gun, and grappling and struggling for balance and position will ensue. Failing an immediate disarm, the only advantage you will have is surprise and that you will momentarily be in better balance. You will lose that advantage in about half a second, if you don't follow up immediately and continuously. You do not have time to figure out if he fired and/or if the gun has jammed. If you're not immediately and continuously grabbing, pulling, moving your feet and jockeying for position, and/or seizing opportunities to strike where it will hurt, you should not have done anything to begin with. He has the advantage of gripping the part of the gun where bullets don't come out and having control of the trigger, and that is not going to be easy to change if the initial disarm failed.
Now if the gun goes off, and you know it's jammed while you are struggling, and you have time to process this, you have more options. If he's strugging to create distance, you might find yourself in a position to suddenly let go and give a shove in the process. Hopefully quick and hard enough to make him lose balance and fall down. You might be able to draw and point before he regains his balance and figures out to rack his gun. You would have to know what you're doing in advance.
