IF YOU HAVE THE PRECISION AIM UNLOCKED:
1. Turn it off.
2. Quit the mission, then start it again.
If that doesn't work:
1. Turn it off.
2. Quit the mission.
3. Restart your game.
4. Start the mission again.
IF YOU DON'T HAVE THE PRECISION AIM MOD UNLOCKED:
1. Participate in any gun range activity.
2. Turn Precision Aim on.
3. Start the mission and then quit.
5. Close the game.
6. Verify the integrity of the game files.
7. Launch the game.
8. Start the mission.
9. Enjoy :)