And if I'm hearing you correctly, you are asking for the bind which enables you to look down the barrel of your weapon? Well, that's not possible in this game. If you're really interested in the iron sites features, I know there are a few good modifications for other games that has that as a feature.
http://www.truecombat.com http://www.planetquake.com/dday http://www.operationflashpoint.com

The last site is not mod, but rather an actual game. It's available for trial on their site. The first one is a mod for Quake, and the last is WW2 mod for Quake2.