Go to the level Narrows or any level level that has a grav lift.
To use this trick you will have to be on forge.
Press up on the D pad and then press the X button to get to the forge and press the RB and LB to switch the things you can buy, go to the list of extra things and go down to the grav lift and get it.
Use the RT and right analog stick to rotate it and then get to a wall that has a gun and make the grav lift that it is either pointing up or angles to the top of the wall.
Press Y to get the other gun and have it thrown into the grav lift and the weapon will either stick to the wall or the ceiling.
Also if you carry around the grav lift, use the RT and anolog stick to rotate it so that any incomer that wants to kill you will be blown away.