Washington residents will face more scrutiny on the purchase of semiautomatic rifles.

Described by both supporters and opponents as one of the most sweeping new gun laws in the country, Initiative 1639 will put new restrictions on the sale of semiautomatic rifles and required guns that are kept in a home to be safely stored.

It was was easily passing Tuesday night despite opposition in many Eastern Washington counties. But it was narrowly passing in Spokane County with about 52 percent support.

Although the ballot title for the initiative refers to “semiautomatic assault rifles” the actual text of the measure makes no distinction between those rifles with certain military features like pistol grips or flash suppressors, and more standard hunting rifles.

So-called assault rifles – a general term for AR 15s and similar firearms that gun-rights advocates scoff at – have featured in many mass shootings, including the recent Tree of Life Synagogue massacre in Pittsburg.

Starting in December, a buyer of any semiautomatic rifle will have to be 21, rather than the current age limit of 18, and pass a more thorough background check. Supporters said the change merely puts the rifles on par with handguns, which have similar requirements in Washington. Opponents said it’s unconstitutional age discrimination against people who are old enough to serve in the military, marry and sign contracts but won’t be able to buy a rifle that can fire a round each time the trigger is pulled. Single shot, bolt action and lever action rifles aren’t covered by the initiative.

Safe Schools Safe Communities, raised about $5.3 million overall with big donations from the late Paul Allen, former Microsoft CEO Steve Ballmer and his wife Connie, venture capitalist Nick Hanauer and his wife Leslie. The National Rifle Association backed an opposition campaign that raised about $600,000, and several other groups mounted smaller efforts.

The campaign easily collected more than enough signatures to qualify for the ballot. But I-1639 had a bumpy road to get to voters, with a pair of challenges by opponents that the text of the changes to state law weren’t properly printed on the petitions.

Secretary of State Kim Wyman said there were questions raised about the petitions but she had no authority to reject them. Thurston County Superior Court judge agreed with opponents and briefly blocked it from the ballot, but the state Supreme Court over ruled that decision and sent it to voters.

Opponents promised to mount another court challenge if I-1639 passed.