Games that allow switching shoulder of weapons, or camera (if it's a third person game - Max Payne 3 for example) tend to have a toggle button (middle mouse button usually).

Right now from far right stance, you need to press CTRL+A to go to middle, and CTRL+A again to put the weapon to left shoulder. If you could add a keybind in controls just to toggle it from left to right, and right to left with 1 press, it would be extremely helpful, and much appreciated.