Given those circumstances, I'd have to say the S&W 500 10 1/2".
This pig is the closest thing to a rifle avaliable! The accuracy and stopping power should allow you to start firing effectively well before your opponent does. I've seen videos of this guy on a rifle range getting decent groups. He actually held the monster like a smg.
Also if your opponent is hiding behind a barrel or another obstacle this is the handgun that has the best chance of penetrating it and hitting him.