We sometimes hear about law enforcement agencies tracing a crime scene gun back to the purchaser. We have also heard about ex-NYC Mayor Michael Bloomberg's anti gun taskforce using firearms trace data to prove that guns which were sold in other states sometimes ended up in NYC.
How would either of the above circumstances even be possible if serial number data from form 4473's stayed with the FFL who sold the gun, unless that FFL has since went out of business and the records were returned to ATF?
As for the original question, the answer can be found here on one of the FAQ pages from the BATF website:
https://www.atf.gov/faq-page#t316n11076I couldn't get a link directly to the answer. Go to the BATF webpage, and then click on Frequently Asked Questions in the lower left corner of the main page. Then, under "Firearms" click on the "General" tab and then click on the question, "Do antiques come within the purview of the GCA?
Somewhere within the FAQ section, there is also the definition of an Antique, and other guns such as black powder replicas, which do not require an FFL transfer.