An ultra-realistic game or mod would be a game that you could play only once. Then when you’ve just joined the game, and you’re killed while still messing with controls, the game would kick you out of the server, ban your cdkey, and hopefully uninstall itself from the computer. That’s death in a realistic game.
Maybe it could employ some training/briefing stage before, so you know what to expect and you can get used to the weapons and controls. On this obligatory training stage, most of the time would have to be spent guarding something or doing chores for your superiors, say like 200hours of that. Then when you join a real game with a real battle going on, your sensitivity, your speed and your weapons will be slightly different due to the psychological effect of being in real combat so you’ll feel completely lost anyways and probably die soon with a shot to the head. War is hell, soldier.