One of the best drills around is to put the weapon on a duty belt the way it would normally be carried and gear up. Gloves, mouthpiece, and headgear if you want it. The guy with the gun needs to defend the weapon against the guy without the gun. No rules on how you have to take it, or how you have to protect it.
What one realizes quickly is :
- Going for the gun doesn't work as well as hitting first and then going for the gun.
- Defending the gun without hitting is not as effective as protecting the gun with one hand/arm and giving him hell with your free hand/arm/elbow/etc.
- You can defend the gun best by simply making sure it can't leave the holster. Sometimes, that means ignoring your opponent's grip and just pushing the gun downward into the holster. Other times, it means grabbing the bottom of the holster and mashing that damned gun into your own side by lifting the holster up as hard as you can.
- If someone is going for your gun, they mean to kill you. Act accordingly. End all drills with drawing your weapon and firing at any threat that's still aggressive.
- It helps if one doesn't get too weapon fixated. As the guy trying to get the gun away, you can often succeed by doing things as seemingly silly as unbuckling the guy's belt. It's pretty easy if the belt is one of those plastic, nylon duty belt types, and the look on a guy's face is priceless when he's defending the gun and you run off with the whole damned belt. Likewise, if the opponent is trying to steal your gun, it helps if you have the presence of mind to reach up and poke him in the eye with whatever hand isn't busy trying to retain the weapon.
- Obstacles are your friend. Learn to throw your shirt, coat, or whatever else you're wearing over the gun so it's tougher to get to. Learn when it's best to do something as drastic as just falling down and laying on that side as you struggle to keep control of the weapon. It's not a good first line of defense, but it has its place.
As a good starter experiment, try doing the drill without any hitting first, and then add the gear and contact later. Watch how fast the game changes.
In all honesty, grabbing the bottom of the holster and cranking it upward so the grip of the gun is pressed hard into your side is one of the better defenses in terms of just keeping the other guy from getting your weapon. You have to do it hard, and it hurts if you do it right, but there's just no way that gun is coming out of there.