Red Orchestra 2 is pretty cool. I haven't tried MP yet, but it's ... well, it's more accessible than ARMA, but still a lot more realistic than most FPSs these days. There are no crosshairs for firing from the hip, no counter to let you know how much ammo you have left ( you can check by holding down the reload key, but even that only gives you an estimate, like "over half full" ). I'm pretty sure ARMA beats it in the ballistics department, but it does have adjustable sights, so that's a bonus. And it has a suppression system ( which, incidentally, Battlefield 3 will also have - not sure if they're similar, though ).