Another vote for Rainbow 6 Vegas here. It's very realistic, you have to use cover and move carefully, as it only takes two or three shots to kill or be killed. The only thing that game lacks is an AR-15 type weapon, and a ballistics model, as anything other than glass will stop bullets.
Day of Defeat is also a cool game; I dont know of any other shooters that let you tote around an MG42 or a Browning 1919, or have deployable support weapons of any type actually, and they are truly deadly if used properly.
Another very cool game that I just finished playing is called S.T.A.L.K.E.R for the PC, it's got a lot of different weapon variants and lets you use iron sights and has some great damage modeling and graphics, I highly recommend it to anyone who is interested in shooter games.