SAN FRANCISCO — Reviewing another round in the gun rights battle, a federal appeals court offered few clues Monday on whether it will uphold a Sunnyvale law banning possession of large capacity ammunition magazines.

A three-judge 9th U.S. Circuit Court of Appeals panel heard about 40 minutes of arguments in the legal challenge to the voter-approved Sunnyvale law, which is being closely watched across California and elsewhere as a measure of how far local gun regulations can go without running afoul of the Second Amendment.

Erin Murphy, a National Rifle Association lawyer representing Sunnyvale gun owners in the case, urged the 9th Circuit to overturn the law, calling it “extreme.” She noted that there is no dispute ammunition ownership is protected by the right to bear arms and that such magazines are commonplace in the home.

“Once something is within the scope of the Second Amendment, flatly prohibiting it is not an option,” Murphy told the court.

The judges did press Murphy on whether the Sunnyvale restriction would be warranted to reduce violence and mass shootings, such as the Aurora, Colorado, theater killings in 2012.

“We have to look at how intense the burden (on gun owner rights) is,” 9th Circuit Judge Johnnie Rawlinson said to the NRA lawyer.

Sunnyvale’s lawyer disputed the NRA argument that the law is intrusive, saying it is an “incremental” step to protect public safety. “It does nothing to dispossess people of arms,” Roderick Thompson, the city’s lawyer, told the judges.

Responding to worries about gun violence, Sunnyvale voters last year approved Measure C, which bans magazines that can hold more than 10 rounds of ammunition. A San Jose federal judge earlier this year upheld the ordinance, concluding that while ownership of such ammo rounds is protected by the Second Amendment, the city’s regulation is constitutional because it is narrow enough and justified by public policy reasons.

The 9th Circuit’s ruling is expected to have sweeping ramifications for the unfolding legal battle over the boundaries of local gun regulations, which have been challenged in courts around the country in the aftermath of a 2008 U.S. Supreme Court ruling strengthening the right to have a firearm for self-defense.

The gun lobby maintains that the magazines banned by Sunnyvale are needed for self-defense in the home while gun control groups consider them unnecessary and a tool for mass killings.

Other California cities have already moved to adopt similar restrictions on the larger capacity ammo magazines, including San Francisco, Los Angeles and Mountain View. California law since 2000 already has banned making, selling, giving or lending magazines that can hold more than 10 rounds, but laws such as Sunnyvale’s go further by making it illegal to possess them in the home.

Howard Mintz covers legal affairs. Contact him at 408-286-0236 or follow him at Twitter.com/hmintz.