The legislation would have banned future sales of most semi-automatic rifles that accept detachable magazines, part of a firearms package approved by state lawmakers in response to mass shootings in other states.
It was lawmakers' latest attempt to close loopholes that have allowed manufacturers to work around previous assault weapon bans. Gun rights groups had threatened to sue if the semi-automatic weapons ban became law.
"I don't believe that this bill's blanket ban on semi-automatic rifles would reduce criminal activity or enhance public safety enough to warrant this infringement on gun owners' rights," the Democratic governor wrote in his veto message.
He also noted that California already has some of the nation's strictest gun and ammunition laws.
The legislation was among 17 gun bills considered by the governor as he works toward a Sunday deadline to act on bills sent to his desk last month. He signed 10 firearms bills into law while vetoing seven.
Democratic Senate President Pro Tem Darrell Steinberg, who proposed the rifle restrictions, said in a statement that more than 1,100 Californians have been killed with guns since the mass school shooting in Newtown, Conn., in December.
"I believe aggressive action is precisely what's needed to reduce the carnage in our communities, and to counter the equally aggressive action by the gun industry," Steinberg said.
The bill sought to ban the sale of assault rifles, but Brown objected that it also would have applied to low-capacity weapons commonly used for hunting, firearms training and target shooting, and some historical and collectible firearms. Brown also didn't want thousands of legal gun owners to have to register their existing weapons as assault rifles and be blocked from selling or transferring the weapons.
"That was, without a doubt, the most egregious piece of anti-gun legislation ever brought to a governor for his signature," said Clint Montfort, an attorney with Michel and Associates, West Coast counsel for the National Rifle Association.
"We appreciate that the governor has respected the rights of California gun owners."
Montfort said the NRA is examining the bills that Brown did sign into law to see if any merit legal challenges.
The governor signed a measure from Assemblywoman Nancy Skinner, D-Berkeley, which bans kits that allow people to turn regular ammunition magazines into high-capacity magazines, as well as two other pieces of legislation that restrict the ability of mentally ill people to possess firearms.
Brown approved a measure making California the first state to impose a statewide ban on lead bullets for all types of hunting. Hunting with lead bullets already is prohibited in eight counties with endangered California condors. About two dozen states also have partial bans, most in sensitive wildlife refuges.
But Brown rejected a bill that would have required owners whose firearms are lost or stolen to promptly notify law enforcement. The governor noted he vetoed a similar bill last year and still doubts that criminalizing the failure to report missing weapons would help law enforcement track down gun traffickers or those prohibited from owning weapons.
Nick Wilcox, whose daughter was killed by a gunman during a 2001 Nevada County rampage, said he was hopeful Brown would have approved more.
"It's a step forward. It's not as big a step forward as we would have liked," Wilcox said on behalf of the Brady Campaign to Prevent Gun Violence.
Paul Song, executive chairman of the Courage Campaign, an advocacy organization that supported the gun bills, said in a telephone interview that Brown appeared to be trying to defuse a possible campaign issue as he runs for re-election next year. The organization later released a much stronger statement accusing the governor of "cowardly behaviour" and saying he "will have blood on his hands."
Sam Paredes, executive director of Gun Owners of California, said gun owners' rights groups will consider mounting recall campaigns or election-year challenges against Democratic lawmakers who voted for the gun bills. Final votes on the legislation occurred last month, just as two Colorado state lawmakers were recalled for supporting tougher gun laws in that state.
Paredes predicted lawsuits challenging bills that require safe storage of handguns; against Skinner's high-capacity magazine bill; and against legislation requiring that buyers of rifles and shotguns pass a safety test.
Still, he said, "We were only shot in the heart 6 times instead of 12 times, and I guess we should be happy with that."