No combat simulators exist on the market. Maybe for fighter aircraft. But the experience of playing a popular FPS is miles apart from even paintball let alone actual combat. It does not teach marksmanship, it does not teach athleticism, it does not teach how to manipulate the firearm effectively, i.e. in every FPS ever you reload by pushing a button. In reality you reload by executing a rather complex sequence of operations which the game is mute about. You can’t even learn about firearms as an abstraction in games because even games which purport to have real guns in them have firearms which do not look, sound, or behave as their real-world counterparts.
An average FPS is about as accurate about how firearms work as an average Hollywood movie, i.e. not even slightly.
The only combat-relevant skill I can imagine FPS games teaching is small unit tactics and communication. No attack has been the result of such tactics.
 Actually, currently the most popular FPS games feature fairly cartoonish settings and are even less combat simulators than what I describe in this comment.
 Except Reciever which is an obscure arty game and not relevant to this discussion.