The NAS NRC "Firearms and Violence: A Policy Review" NAP 2004 pointed out in reference to the effectiveness of gun buy-back programs that there were 6,500 handgun murders in 2003 out of an estimated 65 million handguns, or 1 of 10,000.
Then, the earlier NIJ felon survey showed that criminal sources for guns included theft, burglary, fences, dealers in stolen goods moved interstate, other criminals, friends, relatives. The closest you could get to a criminal getting a gun from an FFL was if they had a girlfriend or boyfriend with a clean record buy the gun for them and that was a minor source of crime guns. Wright & Rossi Armed & Considered 1986, 2008.
Theft, bribery, extortion, corruption of military and police have also been sources of criminal weapons other than FFL gun dealers.
This implies to me that checking FFL gun sales records is like looking for a needle in a haystack.