Moddable Weaponry?

I don’t know if this has already been uploaded, but I’m looking for moddable weaponry like in Borderlands. Where you can add silencers, auto-fire mechanisms, extended mags, etc…
It’d be really cool if someone could make/direct me this…
I’m sure other people would appreciate it, too…

You can’t add extra stuff to your gun in Borderlands unless you use a save editor, and I am quite sure this would be impossible to do with the source engine.

Fretta has an entire gamemode functioning on this concept, actually.