California has a system known as DROS—Dealer’s Record of Sale which functions as a background check form, reporting system, and gun registration system. This system, in various forms, has existed since 1917. California registers all firearms purchases or legally transferred through the DROS purchase/transfer system.
 
California DOJ participates in the federal National Instant Check System (NICS) which is run by the FBI. California is full Point of Contact state for NICS, meaning the state, rather than the gun store, actually contacts the FBI to run the actual background check. What the gun store does is contact Sacramento, who then contacts the FBI, where a human runs a computer search. In other states, gun stores contact the FBI NICS unit directly.
 
What if my DROS background check got denied?
First, check this DOJ page and scroll down to “What You Should Do.” You will receive a letter why from the DOJ in about two weeks. If you feel that you may be a prohibited person (and innocently unaware of the fact), DOJ may seize your guns at some point in the future. Contact an attorney.