He will announce a proposed ballot initiative Thursday that would require background checks for ammunition purchases and ban possession of large-capacity magazines.
Newsom's proposal comes amid renewed national focus on guns, two weeks after a shooting at an Oregon community college where a student killed nine people.
Gun control has also been a top issue in the Democratic presidential primary:
Newsom will make his announcement in San Francisco near the site of a 1993 gun massacre that helped spur federal restrictions on assault weapons. He drafted his proposed initiative with the San Francisco-based Law Center to Prevent Gun Violence, which was created after that incident.
He was not available for comment Wednesday, said spokesman Dan Newman.
High-profile ballot measure campaigns can help bolster a candidate's visibility. And because of dismal voter turnout in the last California election, the threshold to qualify measures has been dropped to 365,000 petition signatures, much lower than the previous standard.
Newsom is also working to legalize marijuana for general use. Ballot measures for next year are already in progress, and Newsom plans to endorse the effort.
California already has some of the toughest rules on firearms in the country. But even though buying, selling and manufacturing large-capacity magazines are illegal in the state, possession is not. Under the measure Newsom will propose, thousands of such magazines would need to be taken away.
Owners would be required to sell them to a licensed firearms dealer, take them out of state or turn them over to police. Possession would be a misdemeanor.
Newsom's proposal would also beef up rules for ammunition sales and purchases, applying the same rules that already exist for firearms. Someone buying bullets would be subject to the same background check as someone buying a gun.
Similar proposals were introduced in the Legislature two years ago but did not pass.
There would be further changes under Newsom's proposal. Gun owners would be required to notify law enforcement if their firearm was lost or stolen, a provision intended to help track loose weapons.
The state would also be required to share data with the federal government on Californians who are prohibited from owning guns. Right now such information is provided voluntarily.
In addition, the proposal would strengthen policies intended to prevent people with criminal records from having firearms. After a conviction, defendants would be given up to 14 days to turn over their guns by selling them to a dealer or surrendering them to law enforcement.
Courts and probation officers would be empowered to enforce the rules.
Follow @chrismegerian for more updates from Sacramento.
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