As I continue to mix my Uechi in with contemporary weapons, I have been looking for answers to some tough questions.
A handgun disarm must work if attempted, and the tough one seemed to be when the assailant had a two hand grip on the gun. Even with practice, it was difficult to pull off.
I've had Maloney sensei's tape on the subject since camp a few years ago, so I went back and watched it several times. It did contain the answer. Instead of both hands palm down, the second hand in needed to be palm up to allow a tighter grasp and upward rotational movement. In fact, the template is the ending of Seisan kata. For a right handed shooter - left hand palm down and the right hand palm up. The mirror is done for a left hand shooter.
Last night I gave it a try. We have a 6'4" 300+ pound student (does judo as well) who was willing to help me out. Even with him knowing that I was going to try to take the gun (fake) away from him, I was successful four out of four times! He was impressed. (I was pleasantly surprised myself.)
By the way, keep the shoulders down and elbows (Sanchin?) in to incorporate your entire body into the movement.
There is another disarm move in Seisan as well but I'll wait and see what others may think of before I mention it.