1. Are you guys SURE this is illegal? Because I thought that that is EXACTLY what they are allowed and expected to do with 4473s when a crime occurs - go in and get the info they need to track down the criminal. I.e. crime was committed with an AK yesterday; go look and see who all bought one;
No that is not how it works.
Firearm is recovered after a crime, or the serial number is read in surveillance footage, or is visible in a picture on someone under investigation etc.
They then use that serial number to contact the factory or distributor and find out where that specific firearm was sent to be sold.
They then call or show up at that location and request information on that specific firearm with that specific serial number.
They then know exactly who purchased that firearm from the store.
That is how it works, and that is closer to how they are supposed to do it. Tracing a firearm that has been used in a crime.
Not building a database essentially registering all owners of certain firearm types just in case one may be used in a crime. And not obtaining all firearm records from some region or area just to create a database and fish and see if one of thousands of purchasers are a criminal they want to find.
That is not legal, though they may choose to argue otherwise.
Of course many local areas have even more details on firearm transactions, some even actually do have registration. In those areas the federal authorities can work with local LEO or state agencies and may use the existing databases.
So in states that require listing serial numbers on licenses, or that register certain firearms when purchased or transferred or keep more centrally located detailed information on transactions, they don't have to do as much work.
In those states they may never even need to contact the FFL to get the information as it is already kept by the state in a much easier to browse format combining all such transactions, like a database or where they keep such state records. And so they only actually need to contact the FFL if they wish to deal with them specifically.
The feds may also unofficially obtain copies of those state and local databases. So if your state keeps track of firearms, the feds will likely obtain that information on state residents not involved in the investigation during various investigations on other individuals.
In states with more firearm owner protections that do not allow the government to keep firearm information, they must go to the individual FFL to obtain records.