What are good games in which you can completely customize guns?

I'm not looking for a game where you upgrade the stats but the gun looks the same, I want it where you add a silencer and the silencer is actually there, or you add a grip and it's actually there. Not just you say, I want my bullets to do more damage and thats just it. I want the gun to look different for every single upgrade I do. Just like Army of two the Fourtieth Day, or Hitman - Blood money. Preferable a stealth game like Hitman but it doesn't necessarily have to. Hopefully a game that was made after the year 2003, i don't mind not super graphics but I don't want awful awful graphics. I would like it for the computer as well hopefully.