So okay, the first thing I did after reading Ljas' post was to type "shooting out windows" and "shooting through windows" into google, to see what realism has to say about shooting out windows and shooting through windows and whatnot.
And would you believe it? Youtube has a lot so say on the matter:
A near miss!
But then I found a document that rigourously treated the mechanics of shooting through glass.
The behaviour of bullets
It would seem that if you shoot through glass, the bullet will pass through, but the longer the range you shoot from, the great the penalty to accuracy.
Then while on youtube, I also found....
Walls being shot by an M1.
Remington 870 shotgun verses walls
Now, to be honest, I don't think realism has too much place in games like this. I mean, what do you want, really? For it to be realistic, or for it to make sense? Do you want cover to be as variable as it is in real life? Or do you want to be able to put a solider behind cover and think "he's safe - for now".