don't know if there is one, don't think so. I use an xbox 360 controller, so wouldn't be an option for me. Also, removing the 12x scope or switching to a different scope or sight, one which doesn't zoom, means you won't get that sway or movement when you aim. Feels a little weird to begin with because you can't see the target as clearly but, with the current 12x mechanic (view resets after each shot), it's more efficient. Also makes it easier to track targets when they move.
ps, on another topic, something i've noticed in missions is loot in a cage with a door you can't open, and it's usually pretty good loot. Look for some kind of electrical junction box in the area, might be a few meters away, and shoot it & the door opens.