Getty ImagesThe AR-15 is a commonly used rifleA California judge has overturned the state's ban on assault weapons, describing the popular AR-15 rifle as "good for both home and battle".
Federal judge Roger Benitez said the law went against the constitutional Second Amendment right to bear arms.
Judge Benitez found that the way California defines military-style rifles deprives the state's residents of weapons allowed elsewhere in the US.
Governor Gavin Newsom described the ruling as a threat to public safety.
California Attorney General Rob Bonta said the authorities would appeal against the decision, which the judge suspended for 30 days to allow the appeal to be lodged.
In the opening to his ruling, Judge Benitez wrote: "Like the Swiss Army knife, the popular AR-15 rifle is a perfect combination of home defense weapon and homeland defense equipment. Good for both home and battle."
He said current California legislation banned "fairly ordinary, popular, modern rifles".
"This case is not about extraordinary weapons lying at the outer limits of Second Amendment protection. The banned 'assault weapons' are not bazookas, howitzers, or machine guns. Those arms are dangerous and solely useful for military purposes," the ruling said.
In a preliminary decision last year, the judge said California's complex definition of assault weapons meant ordinary gun-owners could unwittingly end up owning an illegal firearm.
Governor Newsom, a Democrat, described the ruling as "a slap in the face to families who've lost loved ones to this weapon" - the AR-15.
Getty ImagesGovernor Newsom described the ruling as a threat to public safetyJustice Department officials have defended the law, saying that assault weapons were more dangerous and often used by criminals and in mass shootings.
Friday's ruling followed a lawsuit filed in 2019 by a range of gun advocacy groups. They claimed that California law means gunowners cannot use high-capacity magazines on normal rifles without falling foul of the assault weapons ban.
The first assault weapons restrictions were introduced in California in 1989, and the state has some of the tightest curbs on firearms in the US.
The ruling came amid continuing concern about US gun violence, which President Joe Biden has described as an epidemic.
California suffered a mass shooting only last month, when an employee at a rail yard in San Jose killed nine people before turning the gun on himself.
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