It's not a registry because there are no names associated with the serial numbers. FFL's have to track serials and submit them to the feds.
If they didn't have the serial numbers of his guns they wouldn't have been able to track them.
The FFl has your name on that little 4473 form that you filled out when you bought the gun for the standard background check. Post facto registry.
Who are you to want to escape a thugs bullet? That is only a personal prejudice, ( Atlas Shrugged)
"Those that don't watch the old media are uninformed, those that do watch the old media are misinformed." - Mark Twain