Well, it's a long story.
The first part of the problem is that there is a claim that the Bill of Rights, which includes the Second Amendment, is only a bar to federal infringement.
The Supreme Court has never ruled, as they should, that the Fourteenth Amendment extends immunity from infringement to include an immunity from any state making or enforcing laws infringing the right.
The second major problem is that the federal Ninth Circuit Court of Appeals is particularly backward in that it misinterprets the case US v. Miller. In this key case, the US Supreme Court ruled that only weapons useful to a Militia are protected from infringement. That is not what the Second Amendment says, but the Supreme Court was trying to accommodate disarmament.
The Ninth Circuit has ruled that US v. Miller means that only Militias are protected from infringement. The right of the PEOPLE to keep and bear arms is evidently nonexistent.
That means that two key decisions are required to protect California citizens from infringement. The Ninth Circuit must reverse itself or be reversed by the US Supreme Court with respect to the "collective rights" nonsense. And the US Supreme Court must "incorporate" the Second Amendment by recognizing that the Fourteenth Amendment means what it says.
The alternative is the Initiative campaign to put the right to keep and bear arms on the California Constitution.
Neither alternative is going to happen overnight or without considerable work.
OK that helps a lot. What I see happening if governments continue to try to infringe on gunowners rights is that it will just go underground. While some communities may be able to confiscate registered guns, if the guns aren't registered, they will be hidden and not confiscated. All this will do is create a black market for guns. They may make gun ownership illegal but they won't get rid of guns. Prohibition and the war on drugs never stopped what they were supposed to either.
That would not be "incorporation" in the classic sense. The Court long ago ruled that the "privileges and immunities" clause of the 14th basically doesn't extend any of the bill of rights to the states. That was clearly a wrong decision, and it was even based partly on pre 14th amendment rulings that also denied the applicability of the BoR to the states. Recognizing that they'd bit the Big One, the court later used the "due process" clause of the 14th to incorporate the protections of some of the Bill of Rights to the states. The ones they, the Court, approved of that is.
The Court likes to maintain the fiction that they never over rule themselves, but of course they do it all the time. Stare decisis. isn't really.