Somewhere in the new Connecticut law package (it isn't one law, it's changes, additions and deletions to/from a whole bunch of individual statutes) isn't there a prohibition on threaded barrels? Seems to me I've heard friends from Connecticut mumbling and grumbling about that.
I suppose the exception must be a firearm that's already an "assault weapon" under their definition. As a guy on one forum expresses it, "Once it's an assault weapon, does another evil feature make it assaultier?" So it may depend on what you wish to silence, and when you bought it.