California Gov. Jerry Brown has signed legislation requiring hobbyists who assemble homemade guns to apply for a serial number and submit to a background check.

The legislation signed Friday is aimed at cracking down on untraceable firearms known as "ghost guns."

"Ghost guns" are plastic guns that can slip through metal detectors and unregistered weapons that can fall into the hands of people who are legally prohibited from owning firearms under state law.

By 2019, people who own or create homemade firearms will have to apply for a serial number from the California Department of Justice and permanently affix it to the weapon.

Gun rights advocates have argued that the law will have little impact on those criminals who build guns to circumvent current gun laws.

One such incident involved William Mayes of Mira Mesa who was shot by San Diego police on an Escondido street corner in 2013. Mayes was heavily armed with homemade firearms and explosives, including several small devices hidden on his body, officials said.

[G] Suspect in Officer-Involved Shooting Had Homemade Firearms

In Santa Monica, John Zawahri assembled his own military-style assault rifle and killed five people in a June 2014 rampage after he was barred from legally buying a gun in California because of mental health issues.

Santa Monica Shooter's Arsenal Revealed

AB857 by Assemblyman Jim Cooper of Elk Grove joins six other gun-control measures that the Democratic governor signed earlier in the month. Those included bills requiring people to turn in high-capacity magazines and background checks for ammunition sales.