From Reason:

A California statute requires that all new models of semi-automatic handguns stamp the handgun’s serial number in two locations on each round of ammunition. It is possible for a handgun’s firing pin to stamp the serial number onto the cartridge’s primer, which is a disk in the center of the back side of the ammunition. It not possible to stamp a serial number in two locations, as an erudite amicus brief from the Cato Institute explains. Nevertheless, California Attorney General Kamala Harris in May 2013 declared that all conditions for implementation by the statute had been met. Accordingly, all pistol models created since May 2013 are prohibited from commercial sale in California.