Does it have to specifically be semi-realistic, have old guns, be military themed and in first person? Cuz I know some fine twin-stick sci-fi co-op shooters that can fill the same “shoot everything that moves with minimal effort” itch but they aren’t realistic, military themed or first person.

Like Synthetic 1 and 2 or The Ascent. I know of a couple WW2 games also similar but they completely lack co-op (or any kind of multiplayer for that matter) except for Jagged Alliance.