AMK ar. Ok call me :D But i don't know how good the AMK gun is though.
The key is the one to open the secret room in S4. Remember the first valve you need to shake the gun at? Shake or not, a winged-bie drops from the ceiling, then after killing it, you turn 90 deg counter clockwise to face a horde of 'bies. You shoot the back-most guy to prevent him from closing the shutter, then clear the horde and pick the grenade in either one of the boxes on the floor or the lone dustbin. After that, you turn left into a room, and that's when you can shoot the key on the shelf.
Oh, and a note to deadfun, you'll only get to the room with the key IF you shoot the back-most 'bie to prevent him from closing the shutter. Otherwise no key for you =/
My goodness, did I just describe all that o.O
People who believe in "Gameplay > Graphics" are either too poor to afford a PS3/360, or desperate Nintendo fanboys.