Oh barf. This ignores transparency and traceability on the manufacturer's & merchant's part; and if this speculative example were actually to happen, there'd be a firestorm down on their heads (once the traceability'd got verified). It doesn't happen like this, and these "Wull, what IF" scenarios do nothing to strengthen an argument. "What IF the robber has an illegal assault weapon? - And Obama took away my Tactical Nuke??"...
There hasn't been a single challenge since the rule came down, so it's accomplished its real intent: ending the endless talk. Now all that's left is - talk about the talk. Jay's right.