In the Heller decision (2008), the Supreme Court held that under the 2nd Amendment, the government cannot outright and/or severely restrict ban any firearms in common use for lawful purposes. While the Court declined to identify each and every firearm platform satisfying that description (an impossible task), that language undeniably encompasses the AR-15 platform.

Can a state like California still continue to insist upon certain modifications / alterations to the standard AR-15? Perhaps. But no more outright bans while any proposed modifications should be met with a healthy dose of skepticism and absolute strict scrutiny from the courts.