I was told by an FFL holder that there is no national gun registry. The only official record of who bought a gun from an FFL dealer is the 4473 form, which is housed in the records/files of the FFL. It is my understanding that for ATF/Law enforcement to trace a particular firearm, it starts with the manufacture, who has a record of the distributor they sold it to, who has a record of the FFL dealer they sold it to, who has a record of the customer they sold it to.
So, a firearm can be traced back to the original buyer who completed the 4473, most likely. I'm sure they are not able to trace all firearms used in crimes.
Over 90% of what I collect was manufactured in WWI or WWII.
Damn!! Know I've got more work to do around my light poles...
Electrified razor wire works too. Don't forget the claymores.