California Gov. Gavin Newsom is fuming after a federal court ruled his state's law requiring gun owners to undergo background checks to buy ammunition is unconstitutional.

A three-judge panel of the 9th U.S. Circuit Court of Appeals ruled 2-1 on Thursday to uphold a lower court's preliminary injunction against enforcing the law.

Voters in 2016 approved a ballot measure requiring gun owners to undergo initial background checks to buy ammunition and pay $50 for a four-year ammunition permit.

"Strong gun laws save lives – and today's decision is a slap in the face to the progress California has made in recent years to keep its communities safer from gun violence," Newsom, a Democrat, said Thursday in a statement. "Californians voted to require background checks on ammunition and their voices should matter."

Newsom championed the initiative as California's lieutenant governor.

Writing for the majority, Judge Sandra Segal Ikuta, said the law "meaningfully constrains the right to keep operable arms" guaranteed by the Constitution by forcing California gun owners to reauthorize before each ammunition purchase.

She was joined by Bridget Bade.

The dissenting judge was Jay Bybee.

The 9th Circuit applied the text-and-history test set forth by the landmark 2022 Supreme Court decision in New York State Rifle & Pistol Association Inc. v. Bruen, a 6-3 ruling that struck down a New York law requiring applicants for a pistol concealed carry license to show "proper cause," or a special need distinguishable from that of the general public, in their application.

The 9th Circuit determined the background check requirement regulated conduct covered by the Second Amendment's plain text because it meaningfully constrains the right to keep operable arms. The court then concluded the background check is inconsistent with America's tradition of gun regulation because no such law or analogous law was ever enacted before the 20th century.

The majority dismissed California's arguments that the law conformed to U.S. tradition of regulating guns, citing Colonial-era laws or laws written after the Civil War, such as Reconstruction-era loyalty oaths some Americans had to make before purchasing guns. 

"Because none of the historical analogues proffered by California is within the relevant time frame or is relevantly similar to California's ammunition background check regime, California's ammunition background check regime does not survive scrutiny under the two-step Bruen analysis," Ikuta wrote.

In his dissent, Bybee wrote that the decision did not correctly apply the Bruen test, and the logic behind the decision means any gun regulation could be interpreted as a violation of the Second Amendment. 

"It is difficult to imagine a regulation on the acquisition of ammunition or firearms that would not 'meaningfully constrain' the right to keep and bear arms under the majority's new general applicability standard," he wrote.