Unverified. Go to the very first level in the game. Play through it as normal until you get in that elevator takes you to the surface. As soon as you get to the end of the elevator ride, You will see Jack Dark (the guy with the magnum). As soon as you see him, free 1 slot in your inventory and get your fists up (up on the d-pad). Go up to him with your fists and press secondary fire (RB) to steal Jack's gun (His gun is no different than a regular magnum except it has a black handle, a black barrel, and 200 ammo instead of 75.).