SACRAMENTO, Calif. (AP) — Legislation that has the backing of California’s attorney general would close a loophole in the nation’s toughest assault weapons law, making it more difficult for guns to be swiftly reloaded with high-capacity magazines.

Attorney General Kamala Harris and a Democratic state lawmaker are seeking the change in the wake of this summer’s massacre at a Colorado movie theater.

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The state’s regulations allow gun manufacturers to sell weapons in California with magazines that can be removed and replaced quickly using a simple tool known as a “bullet button.” The buttons get around the state’s ban on detachable magazines that can be used to swiftly reload a rifle or shotgun.

Sen. Leland Yee, D-San Francisco, said Friday that he plans to amend his SB249 in the Assembly next week, adding language to make it clear that state law bans rifles and shotguns with easily detachable magazines.

When it passed the Senate earlier this year, his bill would have banned just the use of one variety of “bullet button,” one with an attached magnet that fits over the weapon’s magazine release mechanism. The magnet lets an empty magazine be exchanged for a fully loaded magazine within seconds.

But Yee’s plan for a broader ban on easily detachable magazines ran into opposition. He said he expects support for much greater restrictions after the July 20 shooting in suburban Denver that killed 12 and injured 58.

“It reminded all of us how dangerous the assault weapons are,” Yee said. “There’s an opportunity now to plug up that hole, and I want to take advantage of that.”

The National Rifle Association urged its members to call and email state lawmakers to oppose the revised bill, calling it “a gun ban monster.”

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It warned that hundreds of thousands of semiautomatic rifles legally sold in California since 2001, when the state’s assault ban regulations took effect, would become illegal under Yee’s bill. The organization also warned that thousands of gun owners could face the threat of arrest.

Harris endorsed Yee’s efforts to strengthen his legislation in a statement to The Associated Press on Friday.

“In recent years, the gun industry has developed a loophole that undermines California’s assault weapons law,” she said.

She said the legal change is needed “to restore the law’s original intent.”

Detachable magazines are one part of the legal definition of what qualifies as an assault weapon in California, which Harris’ office says has the nation’s toughest assault weapons restrictions. The state prohibits weapons that have detachable magazines in combination with other features such as a pistol grip and telescoping stock.

Yee’s proposal would give gun owners until July 2013 to have their weapons altered so the “bullet button” could no longer be used to detach the magazine. His spokesman, Adam Keigwin, said a gunsmith could make the change in a few minutes by welding over the mechanism.

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Copyright 2012 The Associated Press.