Are long guns actually registered to individuals, lets say you buy a gun 3 years ago could the police track the gun using the serial numbers too the owner? I don't think they could figure out who owns what guns because if they knew they wouldn't have needed people too register there "Assualt weapons". but my dad thinks that if they have the serial number they can find the owner.

Long guns are not registered, as are hand guns.  That is, Cal DOJ does not have records of who owns what long gun.  However, dealers do keep sales records with your name and the gun's serial number.  If DOJ is so incline, they can get to it.

Far as I know.

Yes, they can track you down in about 30 min.
There is a paper trail that will lead to the last person that bought the rifle from a FFL.

It starts with the manufacture, then to the distributor, then to the FFL, then to you.  The only snag will be if the rifle was bought a very long time ago and the ffl no longer has the paperwork, or if you bought a rifle from a non-FFL that did not keep any information on you.

My money is on the government NOT being able to keep its paperwork in order, especially if some time has passed.....

As has been stated many times before on this forum, when a long gun is transferred in California, serial number info is not on the CA DROS that goes to the state. Serial number info for long guns is recorded on federal form 4473 and retained in the FFL's records. So the short answer is no, it's not like they can access a database and type in a serial number to find the owner. The long answer is yes, all it takes is a phone call to a FFL to get information on guns that he has transferred.

The exception to that is when a FFL goes out of business and turns over his records to BATF. If that particular FFL ever transferred a gun to you, the feds have your info. I heard a rumor that CA DOJ has tried to get access to out of business FFL records to track down unregistered assault weapons, but I don't know if that's true or not. I wouldn't doubt it. It wouldn't be the first time state and federal DOJs have shared information.