1. Press and Hold right mouse button
2. Switch the shoulder mode by pressing "Q" (from right to left)
3. Now press the middle mouse and go in first person mode aim
4. Press again the middle mouse and go in third person mode aim
5. You can check the difference between third-person and first-person crossair/aim are unbalanced.
...or...
1. Press "Z" and put yourself lying down
2. Press and Hold right mouse button
3. move the view to look up or down when you are in aiming mode
4. Press "X" or "C" to get up
5. Press and Hold the right mouse botton, press middle mouse... press again the middle mouse.
6. Now, you will understand that third-person aiming is totally different from the first person's aiming mode.
Game's version: 0.63.147282