For those who think that a filling out a 4473 is registering a firearm, it's not. Here is a step by step of the process that an FFL goes through.
Step 1. Receive gun
Step 2. Log gun into A&D book
Step 3. Sell gun
Step 4. Buyer fills out 4473
Step 5. FFL calls info into NICS for approval before recording firearm data on 4473
Step 6. If approved buyer pays for gun and leaves with gun
Step 7. FFL records gun inventory number & s/n on 4473
Step 8. FFL logs gun out of A&D book.
Step 9. FFL files 4473
Step 10. Retain 4473 for 20 years
As you can see, the firearm info never left the shop. To track a firearm the popo has to start with the manufacturer and follow the gun through it's logistical route to the buyer...the info is not in a database somewhere and the process is rather labor intensive.
lowers are registered in a sense the manufacture registers them as being made with the government and the store selling the gun has a record of who it's sold to. Either way there's a paper trail. You can call it what you want.