I just installed Call of Duty Modern Warfare 4 (on Windows 8 PC), which seems to be working fine. However I somehow cannot get past the very first assignment, completing the indoor firing range. I am able to walk to firing station 1, successfully aim at each target and fire. Even though the rifle fires, nothing seems to result. I get no instruction prompts. After apparently hitting each target I then walk to the exit door, but cannot open it, so I remain stuck inside the indoor firing range.