I love the entire Halflife series, but it's not really realistic.
Soldier of Fortune 2 is far closer, but even that has a ways to go. The models are a lot nicer as well.
The M4 in SOF2 has a slower cyclic rate than it should have in real life, and it is far too easy to fire single shots in auto mode as a side effect. The ballistics of the weapon are simulated fairly well. Anyone hit with a three-round burst of 5.56mm isn't getting back up anytime soon.
My major gripe about the weapons in the game are that they are artificially balanced; meaning, that they are tweaked such that one weapon isn't clearly better than another, even though that is the case in real life.
It's been my experience that one shouldn't judge realism in games by the way the weapons function; they'll always come up short, simply because a true simulation of combat wouldn't be fun. One should judge realism by how the player is forced to act if he wants to succeed. In SOF2, you're forced to take cover, keep your head down, take *very* careful aim, etc, if you want to get anywhere.