It would bring some interesting tactical choices if the players were able to interact with the gun's tripod to change its height (and/or rotate it). I believe Arma 3 / mods already have this.
I found some article that mention four basic poses:
Prone:
Crouched:
Standing:
Anti-aircraft:
I'm not sure if a separate AA position would make sense from the gameplay perspective, but the first two would surely have their uses.