Notes from practice...
Please realize that I have not attempted to take a firearm from an attacker, and that I am only speaking from a knowledge base of training for disarms against brawling weapons (staff, baseball bat, training knife, shinai, etc...)
From a few hundred hours of practicing disarms vs melee weapons, there are two truisms I've learned:
1) Get off the line of attack. (Works better if you can close while getting off the line, but get off the line, or you get hit.)
2) Disarms that provide a leverage advantage for you, and that take the attacker's balance away, work much better than techniques that rely primarily on inflicting pain or creating surprise alone. (If his adrenaline is up, he may not feel much pain, but a loss of balance or leverage will always hurt him.)
So, from my perspective, if you don't get off the line, and take his balance, you are pretty well screwed.