Have you ever played "America's Army" It is also free and a load of fun to play. Its a 1st person shooter as well with a sense of realism that I have never experienced in any other video game before. It has a huge learning curve so dont be discouraged from the start. This game is worth checking out if you have never played it before. The game is really made by the US Army and is still to this day being devoloped so updates still roll in from time to time.
And to top that off, if you dont like America and its Army, you can always play against them as a 'bad guy'
--Click here for game link--