Looks like this is a way to use the Su122 at last....
The gunsight has been a mystery. I have prepared some crew position/action/views/default keys sheets (will be on a different post) so I found this out whilst looking at the Su122.
1. Go into gunner's position Ctrl C
2. Take control of the gun T
3. Open hatch RAlt C This moves the gunner upwards to the top of the compartment
4. Open visor LWin C This opens the visors on the top so that the sight can see out of the compartment (see attached piccies)
5. Nestle to the gunsight LSht T
Bang Bang!!