Unfortunately, I do not have good news for you.
A felony conviction costs you your state and Federal rights. In Unfortunately, the Federal government does not recognize a California expungement or reduction under PC 1203.4, for purposes of restoring your Federal gun rights.
A California pardon based on a Certificate of Rehabilitation and a finding of innocence probably would satisfy both California and the Federal government, but frankly, as they are all but impossible to get, this is a real longshot.
This site will give you a lot of information about the state of the law and the interaction between the California and Federal government in this area, but it wo't, unfortunately, provide you with much hope.
I am very sorry to be the bearer of bad news but you've got a real uphill battle here unless the state of the law changes. There are lawyers who specialize in pardons, expungements and restoration of gun rights, and you might want to speak to one to see your realistic chances of weapon ownership, since they will vary with the facts and circumstances of the case, but the climate is now in favor of stronger rather than weaker controls, and the prognosis does not look good.