Best bet would be Arma series. It is probably the most realistic shooter and I believe they may even use it to some degree in military training. Other then that series, you might find Operation Flashpoint of similar style, however I have heard bad things about that.
Hi Guys ,
Thanks for the comments.
I used to play Novalogics , Black Hawk Down , Joint Operations, Delta Force Extreme.
I liked a lot of the features of these games, despite not being the top of tree at the time.
You could make you own maps, which is really great.
Yes i know of the Battlefield and Cod series, it seems it will come down to one of these.
Neither of the above games were classed as SIMS , more arcade .