If any of you have watched the show, Sons of Guns, they did an episode of a Total Recall gun (i.e. the Kriss Vector) and they have modified it severely. If you have no idea what i'm talking about, click here for more details. Video here too.
The idea of the gun is a double mag weapon that supports 50 or beyond fire rate, i'd think this would be fitting for the Silenced SMG or Scar seeing it's mag capacity is very large, does great damage, and has very little recoil. There is an internal silencer (normally not possible with the Vector, without heavy modification) a rails attached to the silencer. The Double mag is very odd though, they use a pistol grip and combine it all together making it a full on Double Mag weapon. The Prop gun used in the movie also has a flashlight attached making this damn near perfect for L4D2.
Maybe someone will pick up this idea and make it? Who knows? Just a suggestion for modders out there.