On any map or level, you have been granted the power to hide your weapon.

First, the combination to work this cheat is the following: click and hold the left bumper, right bumper, left thumb stick, the down on the d-pad, and the a button (remember that this will only work if you click them all at once and hold them until the cheat has commenced).

After which you can walk around with your gun down but if you want it back up with out shooting the weapon press the left trigger when you have no grenades.