It took the federal government 17 years to create a registry of 300 million automobiles. Keep in mind that most automobiles are already registered at the state level and all that had to be done was petition each state for their records (4 states opted out). As a result of this 17 year effort, it's estimated that only 77% of cars are in the registry.
Now imagine the federal government trying to register the 250 million firearms in the US, most of which aren't registered at a local level. How long would it take? 30 years? 50 years? How many would actually be registered? 50%? 30%?