The case is Duncan v. Becerra, a 2-1 decision written by Judge Kenneth Lee and joined by Judge Consuelo Callahan; the dissent is by Chief District Judge Barbara M. G. Lynn (N.D. Tex.), sitting by designation. A quick excerpt from the summary of the majority:

California’s near-categorical ban of LCMs [so-called “large-capacity magazines,” which is to say magazines that can hold more than 10 rounds] strikes at the core of the Second Amendment—the right to armed self-defense. Armed self-defense is a fundamental right rooted in tradition and the text of the Second Amendment. Indeed, from pre-colonial times to today’s post-modern era, the right to defend hearth and home has remained paramount.

California’s law imposes a substantial burden on this right to self-defense. The ban makes it criminal for Californians to own magazines that come standard in Glocks, Berettas, and other handguns that are staples of self-defense. Its scope is so sweeping that half of all magazines in America are now unlawful to own in California. Even law-abiding citizens, regardless of their training and track record, must alter or turn over to the state any LCMs that they may have legally owned for years—or face up to a year in jail.

The state of California has latitude in enacting laws to curb the scourge of gun violence, and has done so by imposing waiting periods and many other limitations. But the Second Amendment limits the state’s ability to second-guess a citizen’s choice of arms if it imposes a substantial burden on her right to self-defense.

Many Californians may find solace in the security of a handgun equipped with an LCM: those who live in rural areas where the local sheriff may be miles away, law-abiding citizens trapped in high-crime areas, communities that distrust or depend less on law enforcement, and many more who rely on their firearms to protect themselves and their families. California’s almost-blanket ban on LCMs goes too far in substantially burdening the people’s right to self-defense. We affirm the district court’s summary judgment, and hold that California Penal Code section 32310’s ban on LCMs runs afoul of the Second Amendment.

From the dissent:

I cannot agree that § 32310 is a substantial burden on [the Second Amendment] right. Section 32310 “restricts possession of only a subset of magazines that are over a certain capacity. It does not restrict the possession of magazines in general such that it would render any lawfully possessed firearms inoperable, nor does it restrict the number of magazines that an individual may possess.” Just as “[a] ban on the sale of certain types of ammunition does not prevent the use of handguns or other weapons in self-defense,” and “leaves open alternative channels for self-defense in the home,” § 32310 does not place a substantial burden on core Second Amendment rights because it does not prevent the use of handguns or other weapons in self-defense.

I expect the state to seek en banc review, which the Ninth Circuit may well agree to conduct. Granting en banc would require, basically, a vote of the majority of the Circuit to grant en banc review, and then the case would be reheard by an 11-judge panel consisting of Chief Judge Sidney Thomas joined by 10 judges randomly drawn out of the remaining 28 Ninth Circuit judges. The Ninth Circuit right now has 16 judges appointed by Democrats (including the Chief Judge) and 13 appointed by Republicans, though of course party of appointment is an imperfect predictor of views on this question.

I also expect there may be a much more detailed post in the next few days by David Kopel.