Guns are no longer registered at the state level, the only registration per say is at the original point of purchase, and then only for 3 years. There is no ownership registration anymore. You can run a 4473 a dozen times through different shops involving the same weapon and it will still only trace back to the original point of sale. Cops run a serial number with the manufacturer, manufacturer says it was sold to xxxx shop, shop says it was sold to xxxx person if it is within 3 years of the original sale, the buck stops there.
Where do you get this 3 year business? FFL's are required to keep their 4473's for the duration of the life of their business. Upon closure of business, the 4473's are sent to the ATF.
There is no registration at all. Like you said, the police or ATF can trace a gun from mfg to distributor to shop to original purchaser. They can keep going if the original purchaser has any info on their disposition of the gun, if they can be tracked down.
I've never heard of this 3 year talk. I believe an FFL can be made to retrieve ANY records relating to an investigation.