I preordered the game and receive (among other things) weathered 10mm. I just found a silencer mod for it and now, post modding, there is no graphic for the silenced weathered 10mm. All I get is a big red transparent symbol with an ! in it.

Yep, known issue.
Don't do that!

You can use this mod as a workaround until Bethesda/Obsidian issue an official fix: http://www.newvegasnexus.com/downloads/file.php?id=34794