The new gun law bans magazines holding more than 10 bullets. High capacity magazines have been used in mass shootings, like the one at Los Angeles International Airport three weeks ago. That gunman was equipped with five 30-round clips.
San Francisco Supervisor Malia Cohen sponsored the bill.
"If we were able to eliminate that, it's one less tool that a person carrying out a crime will have at their disposal to use," Cohen said.
The NRA is suing the city, saying the ordinance violates the Second Amendment, the right to keep and bear arms.
Larry Barsetti is one of the plaintiffs. He's a retired cop and with the San Francisco Veterans Police Officers Association.
"All you're doing is impacting honest law abiding citizens of San Francisco; the bad guys aren't going to obey this law, they're not supposed to be carrying guns in the first place," Barsetti said.
Barsetti also believes the ordinance is poorly written. He points to a section which exempts law enforcement officers, who can possess high capacity magazines in connection with their official duties.
"Does that mean when I take my uniform off and I put my off-duty gun on and I go home to San Francisco," he asked. "If it's got 11 rounds in it, I'm a misdemeanor? A violator of the law?"
The city attorney's office, which is defending the law, interprets it differently.
"It does not prohibit off duty officers from keeping their duty weapons because those weapons are issued to them in connection with their official duties," San Francisco Deputy City Attorney Christine Van Aken said.
This lawsuit is simply the first salvo.
Voters in Sunnyvale approved Measure C earlier this month, which puts more restrictions on gun owners and dealers.
ABC7 News has learned that the NRA plans to sue the city the first week of December.