ARMA, Rainbow Six games made by Red Storm, Operation Flaspoint, The SWAT games, and the original Ghost Recon and expansions (also made by Red Storm) are the only realistic shooters I know of. Of these the Rainbow Six and Ghost Recon games are the most accessible, they are getting a bit long in the tooth though.
If by realistic you mean games with a high degree of graphic fidelity, Battlefield 3 is a lot of fun, the Co-Op sucks, but you can team up in a squad in the multiplayer.