I know this is a funky concept, but BATFE considers a suppressor a registered firearm. I think BATFE might be a little "on edge" about people taking a registered weapon and modifying it. Think about it:
Let's see, today I'm going to take my 45 acp suppressor and modify it to use with 5.56.....308......(you get the point). Pay one tax, own an unlimited number of suppressors and suppressor parts.....bad idea.
Someone tried to use serial numbers off of registered Mac 10's to create new, more valuable machine guns. That guy is now doing time.