It was challenged in US District Court, with a motion for preliminary injunction (to stay enforcement of the law while the case was under consideration). The District Court denied the motion, which was appealed to the Ninth Circuit, which also denied it. That denial was in turn appealed to the U.S. Supreme Court.
I'd have given it low odds there -- the Supreme Court is unlikely to trouble itself with whether a city ordinance should be stayed for the months necessary to dispose of a case. But in the latest development, Justice Kennedy (who handles emergency motions from the Ninth Circuit) ordered the city to respond by 5 PM tomorrow.This is quite curious. It suggests that Justice Kennedy is quite interested in this case. While it might not be an indication that Justice Kennedy considers the challenge to the ordinance unconstitutional, if he really thought that there was nothing to the challenge, it is a darn odd reaction.
The magazine limit laws are among the silliest of the gun control measures out there, because they are very broad (affecting not just criminals, but all law-abiding adults), and at the same time, because they exempt police officers (and sometimes even police officers with their off-duty weapons), they have two additional problems:
1. If the magazine ban is for public safety, why exempt police officers? Are 15 round magazines suddenly not a public safety hazard when a police officer has them?
2. Why do police officers, and often police officers who are off-duty, get special treatment under the law? We are talking about something that the courts have ruled is a fundamental human right.