Registration

First.  The 4473 is so close to registration as makes no difference.  If you still own the gun you bought from an FFL and still live at the same address "they" know you have a particular gun.

The loophole is you can still sell the gun without telling anyone in most places and you don't have to tell the gunshop you've moved.

Second.  The speed that a trace happens with stunned me.  With ATF being prohibited from keeping computerized records I pictured a days long search for paper records.  Nope.  Cops call ATF, ATF calls SIG, SIG calls their distributor, distributor calls dealer, dealer calls ATF is the SLOWEST progression.  What really happens is cops call ATF with the serial, ATF calls everyone in the chain starting with the manufacturer and working down to the dealer.  Each link in the chain checks their computerized records and tells ATF whom they passed the gun to.  It probably takes less than an hour from the police initiating the trace!

Third.  I recall reading that unless we got a new law (that failed in Congress) that doing traces like this would be impossible for law enforcement.  Clearly that was a lie (which we said at the time and was instrumental in getting Congress to kill the bill).

All of this is in reference to a murder-suicide locally.  Guy shot his family, soaked his rented house in gasoline, lit $600 worth of fire-works then shot himself.