Finally, a new spin on that old "can I take X knife on plane/through security". I believe that firearms and related items (e.g., ammunition) must be declared (if allowed at all) and the proper forms must be signed. Otherwise, the passenger faces possible federal prosecution if the items undergo scrutiny.
On any metallic items being discovered in your luggage: your departure airport may have additional scanning equipment, but AFAIK, it will be obvious and before check-in. The only problem I could see is if you have a dense, obviously shaped object (perhaps a bomb, but more likely a firearm) you were NOT planning to declare. Any knife that won't spontaneously burst or explode should be fine.
On a side note: I know positively of one case, and I think there were at least two more in other states, where butterfly knives/balisongs were found by the court not to be gravity knives. The latch and the necessary manipulation distinguished them from being fully gravity-actuated. That said, butterfly knives ARE illegal in many places under their own heading. If one is not stupid and does not carry the knife on one's person nor manipulates it in public, that person should be fine.