Where's he supposed to shoot a guy - in the arm? That'll just tick him off - even if he can hit the appendage. Shooting the gun out of someone's hand only works on TV.
Cops are trained to aim for the center of mass because anytime a gun is fired, it is presumed to be lethal force. You don't shoot someone you are unwilling to kill.
I don't think I said shoot the gun out of the hand. If a cop did that, it would be far too reckless, and the guy would have to be a better shot than a sniper.