1. Go to Agia Marina (example), choose a house with blue windows
2. place a soldier behind one of the blue windows
3. place the player on the other side
4. shoot the soldier through the window e.g. with a 9mm sidearm or PDW (Sting in my case)
Only assault rifles can successfully penetrate these really poor windows; some shots of the lower velocity gun seem to penetrate but slow down too much or change flight path too heavily.
Any gun in the game should be able to penetrate and hit a target behind such a window.