Supervisor David Campos proposed an amendment to current law yesterday that would penalize gun owners in San Francisco who fail to store their weapons in a lock box or the locked trunk of  a car.

“If you’re coming to San Francisco and you’re a gun owner and you chose to leave your gun in your vehicle, we in San Francisco ask that you take the extra step of ensuring that there will be limited access to that gun,” Campos said at a meeting of the Board of Supervisors Public Safety Committee Thursday afternoon.

The amendment comes after the high-profile killing of Kate Steinle on Pier 14 in July by Juan Francisco Lopez-Sanchez, an undocumented immigrant with a long criminal history. Four days before her death, the gun used in the killing had been stolen from the personal car of a federal ranger for the Bureau of Land Management.

The ranger was on a work assignment, however, and the legislation does not enforce penalties against federal officers if they are on-duty.

On-duty city officers are already required to store their department-issued guns in locked boxes, and off-duty officers were made to do the same when Campos introduced legislation back in September. This new amendment, supported by Police Chief Greg Suhr, would make it a requirement for all gun owners who leave their weapons in a car to secure them.

The penalty for non-compliance would be a misdemeanor charge of up to six months in jail or a $10,000 fine.

“This is a common sense amendment,” Campos said. “This would be the first legislation of its kind in the state of California, and we believe that this is something that the rest of the state can follow.”

Paula Brown, whose son was killed nine years ago in a case that remains unsolved and who frequently calls attention to the case outside of City Hall, applauded Campos for the legislation.

“My son was only 17 when he was shot 30 times with a semiautomatic gun,” Brown said, holding up a picture of her son on a gurney. “This is going to be a great thing for mothers like myself.”

Alan Martinez, the uncle of a 20-year-old killed in the Isla Vista shootings last year, also supported the measure, saying it will take “thousands of changes” to gun legislation to make a dent in gun violence.

He was joined by Richard Smith, a pastor at the St. John the Evangelist Episcopal Church in the Mission District, who said church members often go on “night walks” in the neighborhood and visit the sites of those killed by gun violence.

“We remember, for instance, Benny Martinez, who was shot and killed blocks from my church. Just a few feet from where he was killed, Jose Escobar was killed one year before,” Smith said. “This is not how god intended us to live, with all these fears and all these tears.”

More than 10,000 guns were stolen in California in 2012 and are part of a million guns stolen nationwide since 2010. In the Bay Area, some 380 firearms are still missing from law enforcement agencies alone, a point emphasized by Supervisor Julie Christensen, who said 10 of those guns are missing from San Francisco.

The proposal will see one more committee meeting before going to the full Board of Supervisors, where Campos said he hopes it will pass unanimously.