Instructions
Use the "Up" arrow button on the Xbox controller's directional pad to cycle through all the weapons in your inventory. Continue to cycle through the weapons until you find the one you want to use.
Press the "Right" directional stick on the controller to aim your gun.
Press the "Black" button on the controller to lock onto the nearest enemy to the immediate right of where you're aiming. Press the "White" button on the controller to lock onto the nearest enemy to the immediate left of where you're aiming.