A friend of mine who is one of our black belts and is also in the navy has addressed this issue to classes on numerous occasions. Gripping the cylinder of a revolver tightly enough should stop it rotating, but holding the slide of a semi-auto is a good way to burn your hand. Some people claim you can hold the slide forward from the back. This is a great way to break or lose a thumb. The only safe way I know to hold the weapon itself is to jam a finger BEHIND the trigger, or place the web of your hand, between thumb and first finger in front of the hammer.